Yesterday, I went to the Walker Art Gallery in Liverpool with 75 Year 1 and 2 children and we had a whale of a time! I've never visited an art gallery with children of any age, let alone Infants, so was interested to see how they took it. We split up into 4 groups (17-20) in each group to visit the Gallery itself, following some of their orientation trails, and looking in detail at 3 pictures. It was following up work we'd been doing on portraits, so it was great. The other part of the visit involved going to Big Art for Little Artists, which I would heartily recommend visiting with young children. They loved dressing up as some of the people they had seen in the portraits earlier in the day.
If you are in the North West, the Walker is a great resource. It's free and they do lots of INSET sessions that allow you to get more out of specific pictures when you lead your own group round. Get there if you can!