Instead of just sending them home when they are done playing (however long you feel comfortable letting them stay... or if you don't want them to play at all, you can do it immediately) I would walk them home... Take the initiative yourself to meet their parents... Perhaps they are unaware that their children are actually in your home, and assume they are just entertaining themselves outside. Although, that is some pretty shoddy parenting if their children can, at that age, be gone for so long without any checking in...
When you meet the parents, you can mention to them that while you 'enjoyed' the company of the children, you really can't be taking care of them all the time...
I would also walk them home if they get hurt... (Maybe tuck a band-aid in their pocket, just in case they actually don't have one at home...) Let the mom know child is hurt, and let her deal with it.
You are making it too easy for the parents to be so lazy! Why should they bother to take care of their kids if someone else is willing to do it for free?
Plus... if you meet the parents, you can get a better handle on the situation... You will be able to judge whether the parents have something going on (not an excuse for neglecting your kids... but makes it more understandable and easier to swallow...) or if they are just being lazy bums. If they are simply being negligent, you can start documenting (take pictures of them playing in the street, note the times the children are left unsupervised) and get child services involved if need be.
ETA... The biggest thing that worries me is that the parents don't seem to care that their children are 'missing' or where they are... Heck, if they got kidnapped, when would the parents even realize? When they didn't show up at home by midnight? Seriously, if my FIVE and SEVEN year olds were missing for even an hour, and I didn't even know where they were, I would have the neighborhood and/or police department searching for them! Not to mention that YOU don't know the parents, yet their children are at your house all day... Pretty darn lucky that you aren't a pedophile or someone else who means them harm!