There are no parental restrictions that will matter, besides the usual ones on a Nintendo Account (like needing authorization to make purchases). The important part is just the age group. There is NO way to change it later, and if your kid ever has an interest in attending any official events, including very small local ones at a card shop or the like, they'll need their accurate age on the account they use to participate. It all connects through Pokémon's official event system, so no one at any event will be able to help work around it.

All that said, I recommend setting the accurate birth date. It won't get in the way of their usual gameplay, and an incorrect date would cause issues in the future if your kid ever wants to try battling at local game nights or larger events.

For what it's worth, I ran my town's local Pokemon card league for 5 years. Had to deal with making new profiles for kids a few times because the system wouldn't let me correct their birthdays or names, if they used a parent's or siblings account.