I wish I could recommend this movie more strongly.
It's competently made for its budget, the acting is above average and very convincing, good photography, creepy sound design and production...
The problem with Mum & Dad is literally that the script just doesn't try for much more than the standard horror gimmicks and gross-outs. By this, I mean there is not much going on psychologically here. The characters motivations and back stories are virtually absent and that detracts a lot.
Yes, I know they're sadistic psychopaths and that should negate the need for much depth, but in a film as static as this, it makes for a long plod (even at 80 minutes). In addition, a good part of the film seems to be lifted from the Texas Chainsaw Massacre (at least in structure) and this struck me as odd, since I seem to remember a real-life case in England very similar to this one. Why not go that route instead of the paint-by-Tobe-Hooper thing?
If you're into the classic Fangoria-style horror --- the "OK, here's the gross-out" approach --- and like quality stuff, this is it.
I just expected more. Sorry.