Just returned after travelling with almost 3 year old twins. We brought a TON of additional liquids through security. Keep it "reasonable" and declare it seperately and you should have no problem. I put it in a seperate bin and said, "Additional liquids because of the toddlers." and continued unloading stuff. Don't make a big deal about it to them. Make it seem like you've done it a million times before and this is just part of the process. We didn't have a problem. We brought unopened juice boxes, unopened water, and even unopened Gatorade. You could probably bring some shelf stable milk as long as it's unopened. Think about the kind that they sell at Starbucks. Looks like the same kind of packaging as a juice box but it doesn't need to be refridgerated until it's opened.

Don't worry about "declaring" the grapes but I would worry about how they will taste after 9 hours without refrigeration?

Plan on the TSA making the 18 month old walk through alone. Have one of you walk through first, then send the 18 month old and then the other of you.