While Shri Krishna was a child He performed many pastimes. So also Shri Krishna lied when mother Yashoda asked Him if He had eaten soil:
Lord Śrī Kṛṣṇa replied: My dear mother, I have never eaten dirt. All My friends complaining against Me are liars. If you think they are being truthful, you can directly look into My mouth and examine it.
So to prove His point when He opened His mouth mother Yashoda saw the whole universe inside it. She realized the divinity of Shri Krishna and forgetting the soil eating incident, surrendered to Him mentally
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However, after knowing Shri Krishna was the Lord of the universe, mother Yashoda could never have treated Him like a son and show her affection towards Him. So lord's internal potency Yogamaya made her forget about Shri Krishna's divinity:
Immediately forgetting Yoga-māyā’s illusion that Kṛṣṇa had shown the universal form within His mouth, mother Yaśodā took her son on her lap as before, feeling increased affection in her heart for her transcendental child.
So mother Yashoda considered, loved and treated Him just as her son instead of revering Him as God even after that incident. Even if it was not a normal act to do so, Shri Krishna performed it as a playful pastime.
Moreover, not even a minor action of Shri Krishna is normal. All His actions are divine however significant or insignificant they may seem
every act of Shree Krishna has a meaning.... When the Lord Krishna ate sand which means that he can obtain anything no matter how sinned. When he opened his mouth it meant that. The whole thing he obtains is resting on him cause he is the paramatma.... But he doesn't want this to change any of the future so he made his matha unconscious but it's also been said that the one whomever is not ready to know the high superior knowledge will go unconscious.
