Do what crazyhorse said, but use
kitten food, not regular cat food. It is a good temporary substitute because it is very high in protien, which is what they would be getting alot of on a bug diet. Whatever you do, do NOT give them water. If you look in the back of thier throats there is a tiny hole before the throat, if water gets in there they will get pnemonia and die. Also, squeeze out any excess water from the food so it is not dripping wet.
Do not use kitten food for a permanent diet though because they need more nutrients than that. Look on the internet and you can find some good recipes. They will need to be fed about every 15-30 minutes. If they don't open their mouths, try bumping the boxes a little(it replicates a mama bird landing in the nest) and tap the sides of their beaks with something, this ussually works, if not, they are probably not hungry. Don't worry, they will let you know when they want to eat
I live in the mountains and have cats, so I am used to finding
baby birds. However, if a bird has been bit by a cat, it WILL die. Something in the saliva kills them within 24 hours.