How can I keep my toddler from eating junk food at other people's houses?

I absolutely agree with both responses above. Make it clear to the person who your child will be with what they can and can't eat. Better yet, pack a lunch/meal or provide the food they will need for the time they'll be there. Just be open with the other person, and let them know how you feel about what your child is eating at their house.

Try packing nutritional foods in a little container that the toddler loves and pack it in their own purse if it is a girl or in the diaper bag if you carry one. When junk food is offered, suggest the food you brought instead.

stop visiting people that don't care if your child gets diabetes...