A common living being or soul can be all-pervading within his own limited body, but the supreme living being is all-pervading in all space and all time
"A common living being or soul can be all-pervading within his own limited body, but the supreme living being is all-pervading in all space and all time"
SB Canto 1
While thus being observed by the child, the Supreme Lord Personality of Godhead, the Supersoul of everyone and the protector of the righteous, at once disappeared, although He is dressed by all directions and is unlimited by time and space.
Child Parīkṣit was not observing a living being who is limited by time and space. There is a gulf of difference between the Lord and the individual living being. The Lord is mentioned herein as the supreme living being unlimited by time and space. Every living being is limited by time and space. Even though a living being is qualitatively one with the Lord, quantitatively there is a great difference between the Supreme Soul and the common individual soul. In the Bhagavad-gītā both the living beings and the Supreme Being are said to be all-pervading (yena sarvam idaṁ tatam), yet there is a difference between these two kinds of all-pervasiveness. A common living being or soul can be all-pervading within his own limited body, but the supreme living being is all-pervading in all space and all time. A common living being cannot extend its influence over another common living being by its all-pervasiveness, but the Supreme Supersoul, the Personality of Godhead, is unlimitedly able to exert His influence over all places and all times and over all living beings. And because He is all-pervasive, unlimited by time and space, He can appear even within the womb of the mother of child Parīkṣit. He is mentioned herein as the protector of the righteous. Anyone who is a surrendered soul unto the Supreme is righteous, and he is specifically protected by the Lord in all circumstances. The Lord is the indirect protector of the unrighteous also, for He rectifies their sins through His external potency. The Lord is mentioned herein as one who is dressed in the ten directions. This means dressed with garments on ten sides, up and down. He is present everywhere and can appear and disappear at His will from everywhere and anywhere. His disappearance from the sight of the child Parīkṣit does not mean that He appeared on the spot from any other place. He was present there, and even after His disappearance He was there, although invisible to the eyes of the child. This material covering of the effulgent firmament is also something like a womb of the mother nature, and we are all put into the womb by the Lord, the father of all living beings. He is present everywhere, even in this material womb of mother Durgā, and those who are deserving can see the Lord.