Yep. Mothra was a children's movie. A decent one, but still a children's movie - and not high-quality, "children's-movie-but-actually-pretty-deep-for-adults" Pixar-style children's movie.
Ghidorah is a decently amusing B movie for people who dig kaiju.
Gojira approached something like art, or at least the closest anyone has thus far gotten to art when making a movie about a giant monster destroying everything.
There's nothing wrong with Mothra vs Godzilla being a "children's movie", in fact I'd say it is a very good movie that didn't need to be better or compare to Gojira.
Shin Godzilla also approached it like art. Not everything in the franchise has to live up to '54.