Probably yes, for a year or so. Just don't do what we've done which is exhaust all the local NT places (I mean go so often that you're bored of all of them) and then fail to cancel your membership for some years.
We joined precisely because we used to live near a lot of NT places (we used to be in Marlow), and got a lot of value out of them. I think that's the key - being nearby. Now that we are in Twickenham, we dropped the NT as there's less of interest, and picked up Kew Gardens instead. Obvious to say, but only worth the money if you use them. We did, and loved it. (Family of 4, aged 4 to 11).
Yes, definitely! You only have to visit a few NT places per year as a family to recoup your membership costs, and they have great gardens to run around in, and also nice cafes and playgrounds etc. My kids prefer those to the actual NT houses <no taste>