Hi there how is your play area going? Im a landscape designer and have been designing natural playgrounds for children. I have used mainly native plants of NZ because they are tough and soft and interesting textures. Some that you may have in England are Red Tussock (Chionochloa rubra) which grows to 1.4m and needs no upkeep is lovely and soft great for around swings etc, Hebe Emerald Gem - grows to a 30cm ball, Coprosma acerosa, Muehlenbeckia axillaris, both soft springy plants. I dont know if they are available in England or not though. This is all a very mixed bag of plants , it all depends on sunlight, moisture etc. Dafs are poisonous bulbs but most gardens have them. But here are some more ideas. You could bonsai some conifers. Also Kniphofia - red hot pokers, you can get mini ones with lemon/green flowers called Percy's Pride, that the leaves grow about 50cm can be cut down to renew the next season, but may be too spikey for you - perhaps at the back because the flowers make great torches. Heather, Hops, alpine or european strawberry - creeping with the most sweet tiny fruit ever, Thyme, lawn chamomile, miniature daffodils - Tete-a-Tete, Viola labridorica - variety of purple/ mauve colours available, spread by seed, perrenial and mini, Ajuga Jungle Beauty - grows very quickly so any damage will heal. Geranium macrorrhizum, succulants - you could make a tufa dish, a big round bowl shape one and plant succulants or tiny alpine plants as a special mini garden - using Saxifrage Peter Pan, really tiny tiny miniature dafs, or create a bonsai scene in it.Check all plants are not poisonous before putting them in. You can check this over the internet.