Seriously: chocolate Rice Krispies treats.
Rice Krispies honestly aren't that bad - it's just puffed rice and they're certified gluten free I believe. Instead of adding sugar to the mix, just melt down dark chocolate and mix it together.
Now, the marshmallows are the sticking point. I challenge you to find decent marshmallows - you probably won't to say the least! This could either be a compromise. I'm sure your child could handle a couple marshmallows just this once, and there's obviously nothing wrong with butter and dark chocolate.
Another possible option: cheesecake brownies. These are VERY solid gluten-free brownies, and you can easily replace cornstarch with potato starch or tapioca starch (both are perfectly primal).
Gluten-Free Brownies | David Lebovitz
I don't add sugar - I just melt dark chocolate chips, they're plenty sweet enough. From there, simply whip one bar of cream cheese with a whole egg and a little bit of sweetener like stevia or a couple teaspoons of coconut sugar, plop a few tablespoons on top and swirl with a knife.
Or you could leave the cream cheese and nuts out altogether and just make plain brownies. Nothing wrong with that, especially with some cream cheese icing (cream cheese whipped with melted chocolate chips - that's it!)