You can fill them at a sink, but that's not a great option when you've got ten kids you'd prefer not to have running through your house. I can fill them with the hose, but only because I can squeeze the garden nozzle just a hair to allow a small trickle of water out. (My son can't.) I thought about using a small syringe, but I wasn't sure they'd be able to operate that and hold the gun at the same time, and I couldn't find one anyway. So I needed some way to make one or more slim streams of water just right to flow into the holes on the squirt guns.
This is what I hacked together in the thirty minutes or so before the party.
Step 1: Tools and Materials
Filling Container - I pulled a gallon milk jug out of the recycling bin. 2 liter bottles or juice containers would work fine, too. Anything that'll hold a decent amount of liquid that's easy to drill through. Don't use anything that contained anything toxic since you know kids will ultimately be drinking directly out of this, if not out of the squirt guns.
Fountain Pump - I was lucky enough to have one of these lying around. If you think you'll get a lot of use out of this idea, you can probably pick one up for $10-$15. If you want to go the cheaper, slightly more labor intensive route you can just make a larger port on the filling container and tell the kids to pour water into it by hand.
Tubing - I had some air hose in the garage I'd used for a pneumatic spud gun project so I grabbed a piece of that. You need something that'll mate with the pump outlet (if you're using one). This was 1/2" ID, I believe. You only need 2'-3', so if you don't have any, pick some up when you get the pump, if you're going that route.
Drill and Bits - You might be able to get by just poking holes in the container, but I think the water streams will be cleaner if you actually drill the holes. I used a 1/2" bit for the top two holes to hang it, and an 1/8" bit for the drain holes.
Utility Knife - In case you need to threaten your kids if they try to steal your stuff while you're working.
Twine - Anything cord or rope like you've got lying around. I used some mason's line. You'll have to thread it through the holes in the bottom, so keep that in mind before you pick something too thick.
Step 2: Making the Hanging and Fill Holes
Once the holes were drilled I took the twist tie from the pump cord and made a little hook to pull the twine through the second hole. The piece of twine I cut was about 3' long, since I wasn't sure exactly how high I was going to hang the jug.
Lastly you'll need to cut an opening for the fill hose. I just sliced out a semicircle and bent the plastic over with my finger. This doesn't need to be pretty, just functional.
Step 3: Making the Drain Holes
Step 4: Hanging the Filling Container
I hung the jug about two feet above the surface of the deck.
Step 5: Setting Up the Pump and Filling the Reservoir
The pump itself had little suction cup feet that I stuck to the bottom of the reservoir underneath the jug, and then fed the hose up and into the hole I'd cut in the top of the jug.
After that I cranked up the hose and filled the reservoir about half full.
Step 6: Turning on the Pump and Testing the Filling Station
You can adjust the intake on your pump, or add more filling streams. I chose the latter. Don't worry about emptying the jug; just crank up your drill and go to town. Add a few and see how it changes the level of the water. As long as the water level is below the hole for the fill hose you're golden.