Tell her it died and that sometimes animals and people get sick like plants and you can't do anything to make them better and they die.. and if you are religeous you can go into the heaven side of it and how it is a better place ect ect..

I personally think you are never too young to have things explained to you.. Explain it simply without dwelling on it.. Just say.. The poor Echidna was just too sick even for a Dr to save