If you're letting a three-year-old play with a DSLR, the least of your worries is fingerprints on the glass. Sorry.
Besides, a filter that does what you ask will cost as much as the lens itself, and even then still have a mesureable negative impact on image quality.
Get the child a true child's toy camera, one that's been certified to not be hazardous, and that is cheap enough that you won't even flinch when it turns into a drumstick. And, for the love of your child, please protect him or her from your gear.