Primitives are relatively cheap compared to larger sheep, and a breed such as Shetland does have advantages for your situation. They are very tame as pets for your children (sheep which disappear over the horizon at the first sign of humans are not very interesting to children). Shetlands come in a variety of colour schemes, so make lovely interesting skins, and inevitably make you want to learn to spin to use the beautiful fleeces. Some may be ready for slaughter in the autumn, although many may need to be overwintered to reach a usable carcase size. However, the superb taste makes up for the smaller size.
It wouldn't be my first choice to have sheep sharing pasture with equines or goats, for the reasons given by others. I would not buy broken mouthed ewes - a false economy, as you would simply be buying in problems,vet bills and knacker fees. Shetlands can be bought for about £30 as weaned lambs in August, direct from a breeder. There are plenty of Shetland keepers all over Britain, so there will be some near you, although they will probably have sold their this year's stock by now. By buying lambs, you will have a year to learn about keeping sheep, and to tame and get to know your animals before you go for breeding them. You may find some wethers available which can be grown on for slaughter next summer.