Why didn't Lord Shiva understand that he was decapitating his own child?

As Lord Shiva is himself one of the Trimurti, he should be a त्रिकालज्ञ (trikālajña) i.e., knower of past, present and future. 

In the event that he's a trikālajña, why didn't he realize that he would execute his own child, Vināyaka (Ganesha)?

1) Boon to Gajasura:

Gajasura was a devotee of Lord Shiva and he got a boon that his head would be revered in future as described as....

There are different stories about how Ganesha got the elephant head yet the most far reaching one I have experienced states that it begun from Gajasura. 

To put it quickly, Gajasura approached Shiva for everybody to recollect his head after death, to which Shivji reacted certifiably by discharging him from the samsaric wheel, taking his make a beeline for be later mounted on Ganesh. 

The entire scene was occurring at Mount Kailash, yet I don't know about a particular data with respect to elephant's body after freedom. 

For protection (in the event that connected pdf ends up noticeably inaccessible), I will cite the entire anecdote about Shiva and Gajasura that brought about Ganesha acquiring the last's head. 

Once, there experienced an Asura (evil spirit) with every one of the qualities of an elephant, called Gajasura. He was a passionate fan of Lord Shiva. Gajasura has experienced a serious compensation (or tapas) for a long time to get extraordinary shelters. Master Shiva was satisfied by this gravity, and chosen to allow him an aid. The devil longed that he could exude fire ceaselessly from his own body so that nobody would ever set out to approach him. The Lord allowed him his demand. Gajasura was not happy with that, and proceeded with his repentance. Master Shiva showed up again before him. This time the evil presence requested that Shiva dwell in his stomach. Shiva conceded his demand, and he relocated to Gajasura's stomach promptly. 

Shiva is otherwise called Bhola Shankar since He is the God, who fulfills effectively, and gifts shelter to His aficionados whatever they want without respect for the results. Every once in a while, this made convoluted circumstances, and place Himself into some inconvenience. In the mean time on Mount Kailash, Goddess Parvati, Lord Shiva's significant other, was stressed over Lord Shiva's whereabouts. She hunt down Him wherever with no good fortune. As a last asset, she went to Lord Vishnu, asking for him to discover her significant other. He, who knows everything, guaranteed her that Shiva was fine, and requesting that her backpedal to Kailash. 

At that point Vishnu, the shrewd chief of the grandiose amusement, organized a little play. He changed his appearance into a flute player, changed Shiva's vehicle Nandi (the bull) into a moving bull, and played out a brilliant move before Gajasura. The charming execution of the bull sent the evil spirit into extraordinary euphoria, and he requested that the flute player disclose to him what he wanted. The melodic Vishnu reacted: "Would you be able to give me that which I ask?" Gajasura answered: "Who do you take me for? I can quickly give you whatever you inquire." The flute player at that point stated: "If that is in this way, free Shiva from your stomach." Gajasura saw then this more likely than not been no other than Vishnu himself, the special case who could have realized that mystery and he tossed himself at his feet. Having freed Shiva, he approached him for one final blessing: "I have been honored by you with many endowments; my last demand is that everybody recollects that me venerating my head when I am dead." To satisfy this desire, Lord Shiva separated Gajasura's head and liberated him from the cycle of birth and demise. He conveys the head alongside him. 

Parvati at Kailash came to know about Lord Vishnu's triumph and her better half's arrival. She was extremely glad, and wished to scrub down. She made a kid out of the mixture that she utilizes as a part of her shower, and gave an existence to monitor the passage of her home. She requested that him not permit anybody inside. In the wake of saying this, she went inside to clean up. In the mean time, Lord Shiva returned home. When he attempted to go into the house, the kid halted him at the entryway as he was not able perceive his dad, Lord Shiva. In outrage, Shiva removed the kid's head with his trident (Trishul). Parvati was uninformed of what had happened and gotten Lord Shiva with warmth, for he had returned after a long stretch. Over the span of their discussion, Shiva says the episode at the royal residence entryways and informs her concerning separating the youngster's head. At the point when Parvati caught wind of that, she burst into tears. Shiva additionally felt frustrated about his outrage. With a specific end goal to support Parvati, Shiva conveyed his troops (Ganas) to look for the kid's head. When they couldn't discover the head, Shiva appended the kid's head with that of Gajasura, and restored him. Subsequent to taking a gander at the tyke (Ganesha), Parvati asked Shiva that who will consider this god with the substance of an elephant. Shiva favored the kid, Ganesha that everybody would love Ganesha before starting any work, and individuals will revere him first before whatever other God in any sort of undertaking.

2) Curse to Lord Shiva:

Lord Shiva once killed Surya and Kasyapa (father of Surya) cursed him as described in answer here.

 3) Son of Shiva-Shakti: 

He struck Surya with a trident and this made the entire world plunge into darkness. Surya was the sage Kashyapa's son and so, for this act, Kashyapa cursed Shiva. "Just as you pierced my son's chest with a trident, your son's head will be chopped off one day," said Kashyapa.

Ganesha (Vinayaka) was just child of Prakriti tatwa as made just by Parvati. Master Shiva changed Purush tatwa to Vinayaka from his Trishula while decapitating him. What's more, hence he thoughtfully moved toward becoming child of both Shiva and Shakti.

4) Removing Maila Buddhi: 

Ganesha was shaped from earth/mud by Parvati. So his keenness had additionally ended up noticeably soil shaped astuteness. Subsequently he couldn't perceive the Supreme Shiva and different Gods, Sages moreover. Along these lines, Beheading the head expelled the soil framed judgment and it additionally evacuated just Maya/Prakirti/Parvati shaped acumen.