Do you have room for hedgerows? They are of such benefit to wildlife if you choose the right species, especially if they can join up with neighbouring woody areas. Children can learn so much from hedges and maybe even see some bird nests once the plants have grown a bit. When we planted up our trees, we chose native species, and included a few which will eventually grow into full sized trees, such as oak, ash, Scots pine, beech. We also have smaller trees such as rowan, elder, holly, wild cherry. birch, hazel and so on. For the hedges, these trees are included, well spaced out, but left to grow above the hedgetop, or cut along with the hedges themselves. The main hedge species are made up of 50% hawthorn, with hornbeam, beech, various wild roses, field maple, spindle, and so on - no willow in hedges. We recently planted a dozen junipers in our 'wildlife strip', as they are a tree native to our area but there are hardly any wild ones left.
Many of our trees are from acorns and holly etc which children have grown from seed, then not known where to plant them out. Your visitors would love to do that. Currently we are trying to work out how to get four 8' tall conker trees, in 2' buckets, up here from the south of England. My youngest grandson planted them a few years ago and now wants them to be here. The half grown conker we have was grown by his dad, our younger son, when he was a child - it's lovely to have history attached to your trees.
There is nothing which grows fast enough to become large within a few years except willow, but something like a weeping willow would provide a 'house' for children to play in in years to come.
Something else you could plant would be a willow bower for playing in and picnicking, maybe with a 'secret' entrance - all fun, but you do need to keep them under control - the willows that is.
Would you also want to grow some fruit, such as apple trees, and raspberries, so the children could pick and eat them on the spot?