I love to have a tea party for bridal showers. one of my favourite theme is a "shabby chic" tea party. nothing needs to match, so that can keep the cost down. borrow as many cloth table cloths that you can get your hands on!! i was short on that so i dyed some bed sheets a pastel colour. you can use canning jars for vases, adding a few fresh flowers to each jar for the table.
i have seen speciality tea bags done up into favours........but i bet the guest might just enjoy a fancy wrapped chocolate or candy instead. for door prizes, you could hit the dollar store and do up a few mugs with a tea bag and treat. think of things you do with a cuppa tea........perhaps a note pad and pen etc. keeping with a "ladies ", perhpas some fancy soaps, body spray etc. or for this time of year, a potted plant . sometimes i even use the potted plants on the tables, and draw names for them to be taken home. since you are hosting this brunch for developmentally handicapped........i think lots of prizes, just wee ones would be great, as we all know how excited they will be to win a prize!!!
i would hit the dollar store.........lol
hope some of my ideas might help. good luck with your event