I don't have a lot of advise, but we do have cat's and a cat flap so we had to teach the cats how to do it!
Firstly, I think you may have scared them by pushing them through! So first of all, to get them anywhere near the flap I'd feed them by it, play with them by it etc etc! all good things happen by the flap! Then when they're fine with the flap there, go out side, maybe you'll need OH to hold the flap open, then make loads of silly noises to try to get them to come and play, lots of treats etc! Even if they just stick their head through, click and treat (if you clicker train!) every tiny further step they make, treat them for it! Hopefully they will eventually get the hang of it!
Good luck!