Coating fruit in Oreos

Have you tried mixing a small amount of water, milk, or cream into the filling to loosen it up a bit? Use small additions, because too loose and you won't have the same effect, but you might be able to achieve a thick, coating consistency. It is likely mostly sugar or corn syrup, perhaps even warming it a bit, or using warm water, milk, or cream would help.

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

This answer is maybe only half fitting, but I'd like to put it forward as a frame-challenge type of answer.

My suggestion is to actively decide to not coat the fruit in Oreo filling. The reason behind it is that I suspect that there are no satisfactory methods to do this. Fruit is inherently hard to coat in stuff; the fruit skin evolved such that insects, pathogens, and any moisture-trapping sun-shading dirt will have a hard time sticking to the fruit. Cut fruit exudes moisture, which "washes" any layers of coating away.

It is not entirely impossible to get around that, but you are limited to using ingredients and techniques that work exceptionally well for coating. First, you would need your coating to be quite sticky (used for example in candy-glazed apples), and I don't think this applies to Oreo filling. Second, you would need an amount that is sufficient for a dipping technique, smearing won't really work - and most eight-year-olds don't have the patience to scratch at Oreos until they have a good-sized bowl full of the filling (and hopefully they are not buying the Oreos out of their pocket money). Third, you will need something which is liquid during the dipping, and then air-dries sufficiently well to not drop off, such as warm sugar syrup or melted chocolate. While you can dilute the filling (helping with the volume), it won't solidify afterwards.

My suggestion is to instead turn it into a teaching moment and gently suggest to your child that a designer has to work with the materials available and not against them. Then challenge him to come up with a different dessert which combines the tastes, without presenting engineering problems. With some luck and prodding, he may come up with alternatives like a pie with a cracker crust made from the cookies and a filling where cut-up fruit pieces are mixed with the original cream. Or cutting out perfect oreo-sized circles out of fruit slices, halving the Oreos and adding the fruit as the upper layer. Or gelling a fruit smoothie in a vessel whose bottom is covered in Oreos. Or even dip the cookie in the fruit (puree first, add gelatine, wait to solidify). There are tons of ideas that are doable, just let him discover them.