If you can’t get easy access to trails then a WTF would be a great half-way house kinda replacement for an MTB.
BUT – why not keep the MTB (or get a cheapie) for days out in car with mates when you can go?
Having a young family is NOT a reason not to MTB. I’vehad 3 children and I know it’s hard to find the time, especially if the wife is – how shall I put this – on the more needy side… (that doesn’t mean sexually – it means thinks you should spend ALL your time doing family/house/kids stuff and resents any time you spend doing anything that is not that).
However, my advice is not to get stressed out by it all… balancing family time/kids and for own personal leisure is not an impossible task. In fact, it’s vitally important both for you and the family.. as you need exercise, oxygen blah, blah.
I used to feel guilty about the time I spent riding instead of being with the kids (and I’m talking about going out on the road bike for 1hr … not a 4hr MTB ride!). Then someone very wise showed me how to reframe that issue: not only do we as humans need exercise but by going biking (or running/whatever) I was acting as a great example and role model for my kids.. they would grow up clearly inculcated with the value and importance of exercise throughout their lives.
….and indeed they have. At ages 9, 11 and 15 my eldest is a BMX nutcase, the middle one a talented and competitive gymnast and the youngest an up and coming freerunner/tumbler/circus act!
Short answer: keep both bikes and use ’em whenever you can.