It can be done with Island Transfer Tool, but there are things you need to know before you do this:
1) As
@BigOnYa said, transferring a save file from a modded Switch to an unmodded Switch will likely result in a ban. If it's the other way around, it should be OK. I know my Lite is still working properly and I've transferred its ACNH save file via Island Transfer Tool to the modded Switch.
2) During transfer, the save file on a source Switch will be removed upon completion, while the previous one (if it even existed) on the target Switch will be overwritten with the one from the source Switch.
The only "safe" way to transfer save from modded to unmodded console would be to mod the other console, create emuNAND and restore the save file via JKSV. The Island Transfer Tool isn't needed in that case, JKSV works like a charm.
That would be much better, yes. If your kid hasn't played Animal Crossing from the start, they'll miss soem of the early things you do with your island. I remember when my girlfriend started playing the game on a separate profile and when we saw that she'll be a new resident on my island instead of getting her own.
Now... I'm not sure if connecting the modded Switch and the unmodded Switch for a local multiplayer session is safe for the unmodded one (Probably not? Otherwise, there would be way too many unhappy people.), but I know it works really good between two modded Switches that me and my girlfriend are playing on. There's a lot of fun to be had if two of you play together on either one of the islands (teaming up for the competitions, helping each other with chores and stuff, can be fun!).
Anyway, those were my two cents. Good luck and have fun!