Are they really though? Disney's animated films over the last decade have had some huge hits. I'd say their market position with children is stronger than ever.
Anyways, a whole Nintendo park is basically impossible now. Uni owns the theme park rights to Nintendo IPs now, has already built them into their existing parks (except Singapore) and doesn't have room for a new park at any of their existing locations. It also, from a marketing perspective, would be a really boneheaded move. Sure Nintendo is massive, but not everyone cares about it, a potential new park would have much broader appeal if it had some Nintendo, and some Dreamworks, and some Harry Potter, etc. Basically, if it was Epic Universe again.