We've already got a Halfords Child seat that attaches with a bracket to the upright and places the child in the seat over the back wheel. I'm now looking at buying a mountain or hybrid bike. Some of them we've been looking at have cables fixed to the upright, don't they get in the way of the seat bracket?? Anyone?? Thanks.
It possibly also depends on the size and geometry of the bike. I have a hamax seat on my roadified mountain bike, but because the frame is quite small and got a sloping top tube I've had to have a non standard 'arm' (the metal beam that attaches the seat to the bracket, iykwim).
I'd say wherever possible try the seat on the bike before you buy - I know people who've bought new seats and then bought a new bike to fit the seat!
Short answer is no, especially if you want disc brakes in the rear. Very light frames may not be sturdy enough, either (frame downtube may not be strong enough to take the load). And otherwise, yes, cables can get in the way. There's a lot to it. You must take advice in a good shop.
That's right; not all seats will fit mountain bikes. I took my mountain bike to a local bike shop a couple of months ago to have a child seat fitted, and they told me it was impossible because of the shape of the bike. I sought a second opinion elsewhere, and the second shop found one seat that would fit - Hamax Siesta - but none of their cheaper seats fitted.