My kids have been in daycare for a couple of years and get invited to a lot of parties. The kids are all preschool age. I think about $5 is plenty for that age, they probably won't remember much of the party anyway!
I like to buy bath products such as bubble bath, color tablets, fizzy balls, cartoon soaps, washcloths, rubber ducks, etc etc. Bath items get used up and are out of the way for the parents. Just make sure there are no allergies.
Another idea is a snack bag. Walmart sells character lunch bags for a dollar and you can include a box of fruit snacks, some matchbox cars, pens, crayons, notepads, thermal cup, stickers, plastic utensils, etc etc. Again, ask about allergies before giving foods.
Sand toys, bubble sets, butterfly catchers, wiffle ball, kites, and other outdoor toys are great gifts too!
Hope this helps.....