Loose stool in 18 month old girl

I have been watching an 18 month old girl 3 days a week for 6 months and from the very beginning she has had loose stool.  First I thought it might be because she was still being breastfed, but that has stopped in the past two months.  Her stools are mushy and VERY stinky.  I have a 23 month old daughter who has solid less pungent smelling stools, I know you can't compare them but the 18 month old's stool is very stinky and I usually change her 2 to 3 times a day during the 10 hours I watch her. She doesn't even seem to know when she is having them.  
She also eats a ton.  She has been known to eat a hot and a half with 1/2 a cup of macaroni and cheese, 1/4 cup of peas, and 1/4 cup of fruit.  When she is done eating about an hour later she acts like she is still hungry.  Is there anything that can be done about her BMs?  Her mother wants to start potty training her when I start to potty train my daughter and I am not looking forward to having to potty train with the stool that she has.

She is 24.8 lbs and 34 inches tall.
Thank you for your help,
Katie