I know a lot of people will disagree with me on this, but I wouldn't make them carry anything unless they want to (and then, only if they REALLY want to). They may think they like the idea of having their own bag, but they don't understand what that bag is going to feel like after a few hours in the hot sun. My kids have to carry their own backpacks through the airport (out of necessity) but for a whole day in the parks - no way. I don't even like having to carry anything. My daughters are 6 and 8 as well, and they will get hot and tired enough without any extra weight. Also, straps from backpacks, especially the strings on cinch bags, can get uncomfortable and scratchy after a while. It's also a big responsibility for them to have to keep track of the bag (on and off rides, after eating, during bathroom breaks). Sure, you can remind them (if you remember) but I find it much easier to throw everything into one backpack and be done. No, I'm not a pack mule. But my kids are on vacation too - I want things to go as smoothly as possible.
If you do decide to have them carry their own bags, either stick to your guns and make them carry it all day (and make sure they understand that beforehand), or get smaller bags that you can fit into a larger backpack if they decide they absolutely can't carry it anymore.