Hi there! I'm glad you're looking for some fun, easy science experiments at home with your 5 and 7 year olds. Air pressure and water sounds like an interesting topic! Here are a few ideas:

1. The Balloon Game: You can set up two balloons, one filled with water and the other with air. Then, you can let the kids try to pop the balloons by hitting them with a gentle tap or a light blow of air from a straw. This will teach them about air pressure and how it affects objects.
2. The Soda Bottle Experiment: Take an empty soda bottle and fill it with water. Then, add a little bit of dish soap to the water. You can then have your kids observe what happens when you shake the bottle and then quickly turn it upside down. They'll notice that the soap film forms a thin layer on the inside of the bottle, which is an example of surface tension.
3. The Syphon Experiment: This one is a bit more complex, but still easy enough for 5 and 7 year olds to do with some supervision. You can set up two cups, one filled with water and the other empty. Then, you can use a syringe or a turkey baster to suck up some of the water from the cup and then shoot it into the empty cup. This will teach your kids about air pressure and how it helps to move objects.
4. The Paper Boat Experiment: Cut out a paper boat and let your kids play with it in a sink filled with water. They'll notice that the boat floats on the water's surface, which is an example of buoyancy. Then, you can add some dish soap to the water and let them observe what happens to the boat.

I hope these ideas help inspire some fun science experiments for your 5 and 7 year olds! Remember to always supervise and have fun learning together!