I am always in "favor" of a favor that is usefull. I hate to spend money (doesn't matter if it alof or a little bit of money) on something that will get thrown away. I myself hate getting little plastic toys that I am going to throw away the next day. Also, candy is not always a good idea, you are never sure if the parents will allow it or not.
A few cool things that I have done, and that we have gotten are now on sale. A small sand bucket with shovel are great. You can even put thier name on it, or fill it with bubbles or other things. I know that it isn't pool season, but you can use them in the tub. another thing we got that was cool was a butterfly net from the dollar store. Can use them in the fall for other bugs, but my boys really liked them. A bug catcher is always fun too for lightening bugs and so on.
Crayons and coloring books are always handy too. And the travel magna doodle type thing.
I would rather spend $2.00 on something that will be used, then $1.00 on something thrown away.
for that age group, the kids don't really know what a party favor is and you could go the way of the mom. How about a pack of travel wipes or even a disposable sippy cup with some treats inside are appreciated by the moms. it is something easy for the diaper bag.
good luck