I would definitely say NO to using mantis as prey items for any frog. Preying mantis insects actually are quite strong, capable of lifting and/or fighting off adversaries 10-50times their body mass. Plus pair that with needle edged forearms and you have a perfect situation for injury to occur. Plus there are many other insects one could use to spice up the diet of their frogs/toads.

P.S. A really cool mantis species are the orchid ones - they can be bright pink and yellow, quite stunning actually!