I actually think it's a pretty bad idea to encourage OP, who admits to being an inexperienced walker, to take a 5yo and a sulky teenager up anywhere high in the middle of winter.
PP's might have children who can walk 8 miles at 5yo or marathons at 13, but OP's children don't fall into that category, and taking them up somewhere that will be wet, slippy, cold and windy is a recipe for disaster. The fells in winter are not particularly safe if you don't know what you're doing - the mist can quickly close in and unless you know 100% where you're going and have a map, you can easily get well and truly lost. Which might be okay as an adult with plenty of spare clothing, but it's not a good idea to risk that with a 5yo.
I've previously been up Helvellyn in a storm - the day started out beautifully. Blue skies, sunshine and a gentle breeze. By the time we reached the top, the mists had closed and there was a gigantic storm above us, with lightning striking the ground metres in front of us. I was with my parents who were both experienced hikers and knew the route well, so we got down safely but if we were inexperienced, it would have been a totally different story.
Please don't encourage inexperienced walkers to do high fells in the middle of winter just because you think it's a better choice of activity over DVD's, shopping and pubs.