Hi there
Great question, gave me ideas for my little and I too
Hmmm
Club penguin?
Webkinz?
PBS kids.org/games
Disneyjunior .disney.com/games
Retrosega.com
Petadobtables.com
Eggcave.com
Furry-paws.com
Adoptme.com
And if you have steam if sure you could find lots of multiplayer games on there