In reply to WILLS:
Our wee-un was in a Yepp mini from 9 months and loves it. It's a front seat. To mount on a mtb aheadset it needed a different part (which includes a bolt around the stem). I used a steering tube extender to raise the riding position slightly, which reduced the bandy-legs problem of the seat being in the way of the cyclists' movement. The perspex screen works well.
It's easy to monitor what they're up to when it's a front seat eg. she's worked out how to get her helmet off, and when the child falls asleep they can rest against your forearm.
Thoroughly recommended.
Leonard