weight lose in toddlers

My 2.5 year old was in size 5 diapers and has know graduated down to size 4 and In oticed the other day that she is starting to lose her pants which fit her perfectly a month ago. Is this normal?

My 2.5 year old also has the same problem with his weight. I have taken him to the doctors several times and they didn't recommended anything. I am very concerned with this as he has gone from size 2 pants to size 0 and his twin brother is in size 3 pants. I have tried different ways to cook food and various different food, but he still doesn't eat very much. I do not think that this is normal or healthy. Has anyone else had the same problem and have any successful solutions?