Posted: 24/04/2015 at 07:32
I'd go for native spring bulbs e.g. the native daffodil (Narcissus pseudonarcissus), English bluebells (Hyacinthoides non-scripta) and snowdrops, Cyclamen hederafolium and some native primroses - they'll naturalise and by the time your trees are biger the bulbs will have settled and it'll look absolutely gorgeous. They'll not compete with your trees for water etc as shrubs etc would.
The daffodils and bluebells should be available now to plant 'in the green' which is the best way. There may still be some snowdrops available 'in the green' as well and native primroses are in the 'wildlife' sections of the garden centres at the moment. If you want some little Cyclamen hederafolium corms send me your address via a PM (click the link under your name in the top righthand corner of this page) and I'll happily pop some in the post to you.
Getting smaller trees is a good thing - they'll suffer from less transplant shock and they'll get growing quickly - they'll catch up with anything bigger planted at the same time.