I bet the FF's are too small, at that size they should be eating house flies if not GB/BB flies.
Usually the feeder order is springtails (really small mantid species), Melanogaster FF, then Hydei FF, stable flies, house flies, then GB or BB flies (Green Bottle or Blue Bottle). At which point most keepers will continue to fed them GB or BB flies due to availability; however, depending on supply (or your culturing abilities) next comes deer flies, flesh flies, black solider flies, horse flies, then Damselfly, and lastly Dragonflies.
Although about any fly above the GB and BB fly stage typically can bite, with the exception of the black solider fly.
Those are just common flies, other such as Phorid/humpback flies or other flying insects like Cicada can fit in the list too. As you discovered usually larger mantids are given/supplemented with non-flying/crawling prey such as crickets, cockroaches, moths, butterflies, waxworms/moths, you name it. Lots of choices, and if the mantid doesn't like it, it won't eat it. They also work for smaller nymphs depending on size - such as pinhead crickets (1/8" cricket nymphs) given at the point just above FF's.
Great to see you and your son are excited, it makes it more fun to have someone interested in it with you. If your wife is anything like mine in regards to "bugs", she will act like she doesn't care but secretly will - and if you let/make her chose a name for one of the mantids she'll like them even more.