A couple ways to look at this:
First, it is useful to remember that in the Eastern tradition, children receive Baptism, Eucharist, and Confirmation at a very young age - long before the age of reason which would allow Penance. So validity is not an issue on the basis of order alone.
Second, which sacrament would be invalid? Certainly not Penance. And the validity of the Eucharist is not dependent on the state of those receiving it. Instead, the graces may not be conferred by reception of the Eucharist if the recipient is not in a state of grace, and such an act would be illicit. So depending on the presence of mortal sin, the perfect contrition of the child, and the attainment or lack thereof by the child of the age of reason, the child may or may not be performing an illicit act by receiving the Eucharist without prior sacramental Penance. But regardless, the Eucharist itself would still be valid.
One last thing, when citing the Catechism it’s better to cite the paragraph numbers, rather than the page. There are several different editions of the Catechism with different paginations, as well as online versions that have no page numbers. But they should all use the same paragraph numbers.:tiphat: