I've used magnetic flaps for years but finally gave up - the cost of continually replacing collars and magnets was obscene (seven cats lose a lot of collars) and Pursley now has a permanently bald line around her neck where the collar has rubbed her fur clean off. One cat got his front leg jammed through his collar and had we not been around it could have been very nasty indeed for him.
Basically I don't like collars so I invested in a microchip flap. Unfortunately I didn't realise at the time that these need a constant AC electricity supply so I can't actually install it - what a waste!
What I'm left with is a magnetic cat flap that I've disabled the catch on.
Now everyone can come in and steal the dinner!
OK - practically speaking for your situation, your little cats are the cause of their own problem - are you sure the kitten is a boy? If not, and both are un-neutered females, this sole fact is what is tempting the big boys in and the boys will not give up until both females are pregnant again, which nobody wants! No cat flap can keep amorous Toms away from an entire female in heat. Even if it's just the Mum cat, she will be ready to conceive again by now, and may be pregnant already.
Can you arrange with a local vet to TNR (trap neuter release) the girl (and the boys for that matter) under the RSPCA voucher scheme? Most vets participate and the neuter is free for strays - you just take them in and bring them back if you're happy to act as their carer/feeder?? (Make double sure the vet understands that you are happy to do this or they could end up in RSPCA rescue - and that's another story).
Good luck, it sounds as if you're serious about the welfare of this little pair! Well done.
