Generally when breading you do either do a ‘three part breading’ (flour, eggs, then bread crumbs) or a batter.
The three part breading relies on the fact that dry things will stick to wet things in small layers. (As a thick layer would be all dry or all wet trying to stick to itself)
Batter is just a viscous liquid, which then crisps up when deep frying…. Which you’re not actually doing here.
There’s also the issue of ‘club hand’ when you’re doing a three part breading unless you keep one hand solely for the dry stuff, and one hand for dipping into the egg, so you don’t end up building up layers of breading on your hands.
As you’re dealing with kids, it might be easier to use a bag— put the Oreo crumbs in the bag, drop the fruit in, close the bag, and then shake to coat.
The issue is going to be two things. (1) making sure the filling sticks to the fruit, and (2) trying to keep the filling from sticking to everything else.
And for this, a vague ‘batter’ might be more appropriate. I would try mashing the filling until it starts to soften up, trying to work it until it gets creamy, and then possibly mix in a spoonful or two of a non-dairy whipped topping (such as coolwhip). You would have to experiment with how well it stuck to the fruit to get the consistency correct.
If it were me, I would probably just apply the filling to the cut edges, then dip it in a tray of crumbs. You could also make the filling a little bit runnier, so you can dip the fruit in (like you might do chocolate dipped strawberries), then roll it in the crumbs.
If you wanted something that’s going to firm up more, I might try making a vanilla pudding (if using a box mix, follow the instructions for pie filling rather than pudding if you want spreadable and not dip-able