The bees are getting water - what you see is part of a hive that is on a water-collecting mission. The plants give them a landing spot to drink from.

They are not aggressive when collecting water, and aside from showing them to your niece and telling her that they are drinking water and to not bother them, you don't need to do anything.

You could remove anything from the pond (plants, rocks, etc.) that a bee could possible land on and they will get discouraged eventually.

I assume the hyacinths are blooming? If so, waiting for the blooming stage to pass or cutting off the blooms in the evening when the bees are not active and disposing of them would probably help a lot. I occasionally get bees around my pond because they need the water, but only a few at a time. If that is the case you might have a hive nearby without other good sources of water for them and then you might have to have the hive taken care of. Some will capture them live and "rehome" them to people who keep hives or want them released in orchards or other places.

If you can figure out where they are coming from you can give them a water source away from your house. See this you tube video. http://www.youtube.com/watch?v=4MwIYCjPcC4

I second what lazygardens said. They seem pretty focused on getting to the water, getting some and beating it back to the hive asap. They really don't want any trouble. They use the water primarily for cooling the hive. So you might see more or fewer depending on the temperature and time of day. I've only ever seen like maybe 10 max at a time taking water, normally more like 4-5.

Not sure how big this pond is but what I've done before is place a little shade cloth into the pond so the bees can land there and walk down to the water. They seem to like that. Maybe they feel some protection from getting eaten by fish. Place this furthest away from people. This is mainly to keep the people from freaking out.

Killing these bees wouldn't really change anything because the hive would just send replacements. Killing the entire hive or moving it is only possible if it's on your property. Bees can travel pretty far and we could even be talking about multiple hive. Plus I'd expect a new hive to move back in sometime down the road.

Thanks for all the help guys! I tried to find the hive but when i followed the bees they flew over our house across the street and into the neighbors backyard so i wasnt able to follow them any further, but they seemed to have slowed down as far as how many will be there at once, and they dont seem to be aggressive like lazygardens said. Ill definitely try and put a birdbath or something out front to deter them, hopefully it remains out front though, we live in kind of a bad neighborhood and have had things taken from our front yard before.

Thanks for all the help guys! I tried to find the hive but when i followed the bees they flew over our house across the street and into the neighbors backyard so i wasnt able to follow them any further, but they seemed to have slowed down as far as how many will be there at once, and they dont seem to be aggressive like lazygardens said. Ill definitely try and put a birdbath or something out front to deter them, hopefully it remains out front though, we live in kind of a bad neighborhood and have had things taken from our front yard before.