Kids are notorious for their irresponsibility so I'm going with what seems most low-maintenance, playful, and least likely to accidentally wound , maim or poison the kid. There are several ways to go.
1. Meditite
They're tough, cute, and only eat one berry a day. If your kid forgets to feed it or is a fickle loser Meditie is fine, it'll keep on meditating and still be around when the kid wants to play with it again. Bonus: It can protect the kid and teach it the value of discipline. Similar thing apply to Reuniclus too.
2. Object-mons that don't eat or be cleaned up after at all. Rotom, for instance, doesn't need to eat and can possess the kid's electronic toys. They generally have a happy demeanor and love to play. Klefki could be a fun playmate as well, seeing as it doesn't eat and jingles when you hug it.
3. Gentle plants. I wouldn't let a child near something like Gloom or Amoongus, but something like Whimsicott or Luidicolo would be great. They don't need to eat much because of photosynthesis, they're generally playful and friendly as opposed to being aggressive or toxic.
4. Pokémon that are protective and self-suffiecient. Many pokemon are instinctively protective and as a bonus, can thrive on their own while carrying the burden of looking after your child. These pokemon are more nannies or responsible big brothers/big sisters than playmates, however. Good examples include Chansey, Aggron, Kangaskhan, Leavanny, Pangoro, Golurk, Gardevoir/Gallade, and Braviary.
Pokémon that would be terrible to leave your child with: Chandelure
Drifloon
Gyarados
Nosepass
Sharpedo ("Bully of the Sea" and has the word pedo in its name)
Hydreigon
Tyrunt
Pineco
Primeape
Electrode
Roselia
Toxicroak
Magmar
