My DS2, just turned 2, has been wearing glasses for a couple of months now. We were completely in the dark when it came to choosing which ones as neither dh or I wear glasses so we didn't really know what points to look at! When we actually got in the opticians too, we found it very difficult to tell which ones suited his face as it was such a shock seeing him in them in the first place.
We first went to a smaller opticians and the lady there was very helpful but was insisting that because of his age, he had to have the hooked behind the ears ones. These would cost £20 on top of the free prescription which was fine, but if and when he broke them, we would have this fee to pay every time - so just watch out for that! If you just choose the standard NHS ones (which are nice, not jam jar bottoms like they used to be!) they're free and fixed for free too.
We then went to Specsavers who were excellent. They had a large selection and lots of helpful staff. We were lucky in that we got a prescription for 2 pairs so we chose two different styles, each in a different colour, so that in time, we could decide which we preferred. We didn't get the behind the ear hooks (they said we could get them at a later date if he struggled with them) and it hasn't been a problem. He still rolls around on the floor with DS1 with no harm to the glasses (well not every time anyway!). We find Specsavers really good at fixing them too, we just drop them in and pop back for them. They will get broken or bent regularly, no matter how hard you try so be prepared for regular trips to the opticians. Specsavers have a website if you want an idea of what styles to expect beforehand.
We made a big deal about them being special and he quickly grew proud of wearing them and he rarely takes them off but when he does we just don't make a fuss and leave them off a while. I think he's noticed the difference in his sight too. After his bath, he now says "glasses" as he wants them back on for story time!