A good portion of kids films can be made to seem darker and more adult if you write the synopsis in a bare bones way, here is a few examples

Finding Nemo:
When a man's son, permanently disabled after surviving the brutal murder of his mother before he was born, is kidnapped on his first day at school The fathers only hope of finding his is a mentally unstable woman who leads him on an adventure to find his son before its too late!

All dogs go to Heaven:
A murder victim returns from the dead being not content with paradise and uses an orphan girls gift of speaking with animals to help him gain a fortune in horse racing, but then the stakes suddenly go to high for him when his murderer, a mob boss, finds out about his return and decides to finish him off once and for all, even if it means the girls death

Once upon a Forest:
When a tanker carrying poisonous gas crashes and the fumes leave a young girls parents are left dead and she herself is nearing deaths door from the same gas her friends go on a dangerous journey to get the only known cure leading them to meet colorful characters and dangerous mechanizations.

Feel free to add your own! Just make sure the synopsis you write still fits the film