$ 25 dollars a day for 6 hrs is only about $ 4.15 cents an hour for TWO kids.. I would say your friend is very lucky to have you.
& none the less you also provide food.. which can be expensive since kids each a lot of fruit.
If your friend is working - she should expect to pay - as much as a regular child care center would provide, or just less as it works for both parties.. you get paid and she pays less.. but for the most part regular child care usually includes the parent provides lunch.
so I'm not saying you should stop watching your friends kids..but perhaps have a moment where you two can talk about .ouch. boundaries.
I have watched my friend's children in my home & although it was more convienent to do all the same food.. my friend was thoughtful to provide some of the food for all the kids the day her children were there.. or enough to share. as kids all the same age like to have the same thing.
it's not to much to ask.. and the older children would likely need a snack afterschool and it should be packed in advance of the date.
One wouldn't think the food issue would be a big deal - but it can be expensive.
Lastly.. only watch the kids as much as you feel able and don't be afraid to say when it has become to much... before it becomes.. too much..
hope this helps.
-marg.