They are at a difficult age to tell them - but my guess is that the older two already have an idea what is goig on. Maybe just talk to the older two more openly and canidedly?
My daughter is only 3 but it is crazy how perceptive she already is - she studies my plate and then asks me where certain foods are hat are on her plate but not mine. Obviously at her age she has no idea whats going on, bit one day she will piece it all together.
My daughter is 5 and sounds very much the same. She has also commented how she loves her 'skinny mummy' which shows she has noticed the difference in my body size compared with other parents of kids at school, park, etc, not as though she's actually worried, just saying she loves me how I am-type of thing. As she gets more and more perceptive I can see her becoming worried about it and NOT loving it because it will become clear it's a disorder.