Why not get them to make their own traditional style catapult from a forked stick and some square section rubber that is easily obtainable from Ebay, baring that soft rubber and pouches can be purchased from every tackle shop in the country, as of course are cheap, low powered catapults used for loose feeding.
If you go down the traditional route they are invested in the project from locating and harvesting a simple stick to making something they can use, obviously Dad can do any knife work if you are uncomfortable with them doing it, but should they want to get hands on, then a vegetable peeler works wonders for removing bark.
The three catapults in the picture are some of the ones the boys and i made many years ago, the central one has rubber from the tackle shop and is very low powered, the two others have square section rubber, the pouches are bits of leather cut out with scissors, for these we used cable ties to secure the bands, but of you want to do it properly, then the bands and pouch can be secured with cordage and whipping knots.
All three of these forks are still going strong. albeit on multiple sets of bands, over time the the boys went through these to using more "modern" types with flat bands, right up to tapered double theraband gold, but to start with, these were cheap, robust and ideal to have a little bit of fun with.