Sheelagh, you may already know that the RHS has a whole bit dedicated to school gardens. It costs nothing to join and there are project suggestions, seed trials and all sorts of things.
Don't know what age yours are, mine were key stage 2. Small border forks were extremely useful at certain times of the year, reasonable quality hand trowels and forks were in constant use, shears and secateurs were popular (and useful) as they're the kind of thing children are often not trusted to handle these days. Gardening gloves in the littlest sizes, a couple of wheelbarrows or pull along trolleys, those bendy buckets in bright colours, a compost bin - and we even saved up for a wormery though you need to bear in mind the school holidays and whether it can be looked after. We also raised some of the money for a shed to store it in and the school secretary managed to wheedle the rest out of the head-teacher!
Have lots of fun. I always reckoned that the whole curriculum could be taught through aspects of gardening and the children loved it because it was such a relaxed atmosphere, no pressure.