Mam Tor is very easy, though too small to technically be a mountain. Park in the Mam Nick car park, between Castleton and Edale, and follow the steps up to the summit, then walk along the ridge and back. Absolutely no need for map reading. Although I don't recommend it during a gale, that time I went up was interesting to say the least. If you are slightly more confident map reading, you can descend and find your way back to the car park using the broken road (the old A road destroyed by landslips). But to be honest the Mar Tor ridge is stunning enough anyway that people are usually happy to do and out and back.
Kinder Scout, just across the valley is a mountain, though its a mountain plateau, so the actual top isn't the best place to walk to. I'd only recommend it for beginners though on a bright sunny day, after a couple of weeks of really dry weather, and by sticking to the Pennine Way up and back down. The weather can turn in minutes, it's frequently impossible to see where you're going. The paths aren't clear. Lose the path and your quickly deep in peat bog and lots of the paths turn into scrambles. Lovely mountain, but you do need the right equipment and a bit of a clue.
Snowdon on a nice day, out of snow/ice season (May to middle of September usually OK), when you've checked the mountain weather forecast is doable with kids. If you're not sure stick to the Llanberis path and turn around if necessary. Don't rely on the train unless you have prebooked tickets. There is a cafe at the top (open when the train is running to the top) and a cafe halfway up the Llanberis path. If you're not used to walking though I'd allow 10 hours for the round trip.