This is so much simpler than you may think, does not require lots of expensive equipment and is a geat way to get the children involved in outdoors activities and planning their environment; enabling children to have a say is an important part of the EYFS.
Little Blossoms has been awarded a Gold award for having a wildlife friendly garden by the Cheshire Wildlife Trust. Anyone can take part, wherever in the country you are and they are keen for the word to get around. To apply, use this form; they no longer require photographic evidence though so don't rush round taking pictures of everything!!
Try making bird feeders, building some mini bug hotels; taking some ideas from this site, drill into the ends of a small bracnch of wood and hang up or tuck into a corner, roll up corrugated cardboard and push into an empty plastic drinks bottle with the end cut off, and saw bamboo canes into small lengths, tie with string and hang up around the garden.
There is more information on this page from the wildlife trust. Try searching the web for free/part-funded local and national initiatives to help you get started.