500 pounds over only 10 feet? It doesn't have to be a very heavy bridge at all - one 12" log should do it. Also, you can use a short log as a "mud sill" embedded alongside the creek bank.
I've built several bridges like this spanning creeks from 10-feet to 36-feet. My bridges were built to carry a pack train so were pretty heavy. Here's my suggestion:
1) Two Douglas-fir (or Cedar) mud sills, 12-inches in diameter, 8-feet long, peeled and bedded along the creek banks. Pin them into the bank with 4' long 5/8" rebar stakes (pre-drill holes through the sills).
2) Two 10-inch diameter stringers about 12-feet long (or that will overlap your mud sills by about a foot), peeled. Space them about 2 feet apart. Species doesn't really matter unless you want it to last 100 years. Notch the mud sills and set the stringers in the notches, then drill two holes in the stringer at each notch and drive in an 18" piece of 1/2" rebar. If you want to get really fancy, treat all exposed cuts on your logs.
3) Deck the bridge with 5"-6" logs split in half down the center, laid flat surfaces up. 2x dimensional lumber would probably support people just fine, but untreated lumber gets tender in damp conditions.
4) Put up a handrail or curb logs along the edge to remind the kids that it's wet over the sides.
If you need some pictures, I do have many available...