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.