We use UHT milk all the time at home. It does take a little getting used to though.
I just checked our carton and it says "Not to be used as a complete food for children under 2 years". I take that as not to replace babies milk (formula or breast) with it. But as part of a normal toddlers diet it should be fine.
If you have any questions talk to your dr, or baby nurse or even ring the parent helpline, they may have some advice.