I’m a Pokemon League Judge if you ever need answers for questions just fire an at.
I also own a store so can help with purchases! Rotation hits in April, so if they ever want to play with other kids (and have all the hip cool things) then getting something that doesn’t have loads of Regulation E cards is good (in the bottom left hand corner next to the set symbol/code there’s a letter in a white box, E rotates in April, D has already gone - you can use cards that are reprinted as F/G if you have an E or older version, for example Switch and Professors Research)
League Battle Decks are the best value by far, the new Miraidon one has some VERY playable cards and Miraidon as an archetype is winning regional championships.
The best thing about Pokemon is large chunks of decks will be the same even if you build something new (supporters etc) so you can find a Pokemon your kid likes and as long as it isn’t Charizard EX, Roaring Moon or Iron Valiant you can usually put together a very good deck for <$50.
Use Limitless for deck list ideas, I believe they even have basket options to buy entire decks frOm reputable websites