Sticker Design for kids

Our company sells to doctors surgeries in the UK. As part of our marketing efforts we would like to give away stickers for the surgeries to hand out to kids who have had injections etc. We would like somebody to design 4 types of "I've been brave" stickers for us.
The stickers need to be fun and feature characters that kids will find interesting. At the risk of stereotyping, boys may prefer a sticker featuring a heavily muscled superhero roaring (like the Hulk in the Avengers film...), whereas girls may prefer a fairy. Both may prefer a monster in the style of Moshi. The stickers can be round or square or whatever.
Each sticker needs the words "I've been brave" or "I am a superhero" or similar on them, something as original as possible given the remit. Also on the sticker must appear our website address www.eSuppliesMedical.co.uk .
The final product should be a sheet designed to fit as many stickers on as possible in a format suitable for us to send to a printer to have them printed in volume.
For ideas, this company does a good job: http://www.schoolstickers.co.uk/medical