I’d reccomend that as the adult you spray prime the prints first on your own. Hardware store paint is fine for this. I’d suggest white to go along with the following part of the suggestion.
Craft paints, like Applebarrel or Folk Art are very cheap and found at art stores and WalMarts.
They have a tendency to be thick and also separate if thinned with a lot of water. The solution is to buy some Flow Aid. Liquitex is a cheap and common brand. You can probably find this at an art store, if not it’s cheap online. Buy some cheap pipettes.
When you squirt craft paint onto a dry palette, add 1-2 drops of flow aid with the pipette and give it a couple of swirls with the paintbrush. This will make the craft paint run a little more without separating. Depending on how any given paint acts you can add more paint or flow aid to get it to the desired thickness.
If you’re bold you can experiment with adding flow add little by little directly to the bottles if you want the paint bottles to be ready to use, but you’ll have to experiment to find just the right amount of flow aid to add. I’d say adding too little is better than too much.
The coverage of this paint is going to be a little transparent which is why the white priming is probably better.
Obviously the results will not be as fine as dedicated hobby paints, but for a kids first time painting party event, they will work.