Perhaps he felt the need to release children's songs, as a father who had to listen to the usual junk put out for kids, and he thought, do you know what - I could do better than that. We've all thought it about kids' stuff, but he went and did it. He gets panned for it because everyone expects him to have a certain output, but why shouldn't he give it a go? It worked out quite well financially as I recall....
I love the bit in Shrek (dunno which one) where the old frog king dies and the frog chorus sings Live and Let Die. Cool.