Need advice/recomendation for a bike to carry child

I favour a solo road blast but with the summer getting into full swing the wife would like us to enjoy family rides.

Her family ride cycle trails so my road/track bikes aren't going to cut it for me. The wife is a little wobbly on a bike, so it would be down to me to carry our little one (8/9 months).

I think I would like a hardtail mountain bike but am clueless when it comes to sizing or whats good. I have a pretty good idea of hot/not components but i'm unsure on frames.

I ride a 52cm square road/track frame and i'm 5ft10.

As for carrying the baby, I guess I can use a co-rider seat or stick her in a backpack style carrier. Unless anyone has any better suggestions?

Would also like to use the bike without the child paraphernalia to widen my rides to include a bit of exploring the South Downs.

Anyone help me out?

Can't comment much on the mountain bike part, but having had both front and back baby seats, the front one is better for a smallish child. I have one like this and he loves it.
http://www.ebay.co.uk/itm/QUALITY-BIKE-CHILD-SEAT-FRONT-MOUNT-YEPP-MINI-BLACK-MAX-WEIGHT-15kg-AHEAD-ONLY-/171307095192?pt=UK_SportGoods_CyclAcces_RL&hash=item27e2b2d498#ht_1221wt_993

I would not ride with a kid in my backpack, the seats protect them better and i would be worried I might fall on them in the case of a mishap.

I've used a variety of things, I'll try to give you a roundup.

Rear seat - I used a Halfords one that you can clip off. kids always liked it and it was fairly convenient.
Trailer - Got one off ebay, about £30, kids love it, still enjoy it now, but is cumbersome and useful in quite specific situations.
Tag along - Also off ebay, about £50, son enjoyed it, daughter used to fall asleep on it, have used it to commute in London a couple of times.
Recumbent Tag along - Second hand £85. Daughter has mainly used it, loves it doesn't matter if she falls asleep, wife not keen, wont let me use it on the road under any circumstances.