I also used to have SUCH a hard time saying no more until I had an inspection from my head office one day and I was told I was actually feeding them way too much ! lol She showed me what a regular portion looks like ( 1 fruit, and 1 granola bar are plenty) and I have since learned to say , no more.
We should be trying to show them proper eating habits anyways and portion control is one of them.
Lunch or supper is never too far away anyways