I learned to swim before I could walk, and my mother taught me. It is realistic that you can teach your kids to swim. I was a lifeguard and swim instructor for years during high school and college, so I know a few "tricks of the trade." Here are some pages out of my lesson plans:
1) Have them start by blowing bubbles. If any of them is afraid because the pool seems soooo big, just cup your hands together under the water and say, "Fill my hands with bubbles."
2) Work up to getting them to go under water and blow out all their bubbles underwater. I used to play "underwater circus" with my classes of 2 year olds, where we'd all go underwater holding hands, and when we came up they'd tell me what silly circus animal they had seen underwater (such as a purple striped elephant, or a green tiger). The purpose here is that a child who is comfortable underwater will be able to open her eyes.
3) Once they can blow bubbles and put their faces in the water, you can move on to the "superwoman float" - where they put their arms out in front of them, legs kicking away in the back, and blowing out their bubbles with their faces in the water. You can stand 5 feet from the side of the pool and give them a boost toward the wall.
4) Your little ones will not be able to do any kind of arms above the water at this age - they may do better swimming underwater. Your 6 year old will be able to learn how to move her arms.
5) Teach them to float on their backs. Tell them to "make a pillow" with their hands under their heads (be sure to stand so the sun isn't shining in their faces) and tell them to put their tummies up toward the sky, and relax. It's ok if their feet don't float. Put one of your hands on their back and one hand on their tummy. Once you feel them floating, slowly remove your hand from their back but keep your hand on their tummy. They will think you're still holding them!
6) They should know how to jump into the water from standing on the edge. Obviously you will catch them at first. As they get better at this, have them jump in, turn over onto their backs, and float. This is a great survival skill in case they ever fall in - they need to know not to panic.
7) Once they can jump in, turn over and float, you can work on jumping in, turning around, and swimming back to the edge.
8) Teach them to go around the pool edge, hand over hand, until they get to the steps. (The idea being, they've fallen in, made it to the edge - now can they get out of the pool?)
9) Assuming your 6 year old will master all of this very easily, she will be ready to learn to swim across the pool (with or without arms). Since she will know how to blow bubbles already, when she runs out of bubbles, tell her to lift her head and "take a bite of air." I know it sounds weird, but it gets the idea across to them that they need to take a breath, close their mouth, and put their head back down.
You should be able to do all of that during this summer. What's listed above won't get them onto swim team, but really the goal is water safety above all else. This will get them comfortable with swimming skills and will help them not to panic if they do fall into the water and you aren't right there to catch them. I hope this helps! Good luck and have fun!