Have you checked out Overcooked!? It's more of a chaotic co-op cooking game than a platformer, but it's incredibly engaging for families. The frantic pace and teamwork required to complete orders lead to some hilarious moments. If you're looking for a more traditional platformer experience, Rayman Legends is fantastic too; its vibrant art style and smooth gameplay make it perfect for players of all ages. Both games are super accessible while still encouraging cooperation.
Do you have a preference for 2D over 3D platformers, or is it a mix?