Yes it's normal for kids to hump objects or things in a messy diaper.
Lots of kids in diapers will hump objects in the diaper or even just hump in the diaper when it's wet or messy.
I've seen kids and even my own nephews hump things and hump their car seat strap or highchair strap in a messy or wet diaper.
I like to strap myself in a car seat and hump the strap with my wet and or messy diaper.
Even just grabbing the front of the wet or messy diaper with my hand and moving the wet or messy diaper up an down my penis makes for good fun and good masturbation.
It feels so good to rub the wet or messy diaper against my penis and then explode with an orgasm into the diaper.
Masturbating in wet diapers is even better than masturbating with your hand and you don't need lube either because the wet diaper is slippery enough to work for masturbation.
If you haven't tried masturbating in a wet diaper then you should as you're missing out on a good thing.