H.A.
I live in Oregon City, and I know there is a sprinkler park at Rivercrest park which is not too far off Molalla Ave. My 18 month old son loves it although I think it is better for a little older children. Your 4 year old will probably love it. There is also a wading pool in Gladstone at a pretty nice park. This might be fun for your oldest, but it was a little crazy last year and don't think I would do again till my son gets a little older and I am more comfortable with him around water. Oregon City also has a wading pool at the big swimming pool, and I don't remember ever seeing it that busy so it might be a better option if you want to have a little calmer environment. The only downfall is that it is just a pool, and no nearby park to run around in.
In West Linn there are movies in the park during August. It is the big state park I think that has them, but I can't quite remember the name of it right now. I have never been there, but found it online when I was looking for other things to do this summer. It looked like they were going to have some good ones, and I believe they were on Saturday nights. I saw the website a month or so ago. North Clackamas had movies last year in the park as well, but I haven't seen any information about them this year.
In Oregon City starting next week they have concerts in the park at the Carnegie Center every Thursday starting at 6:30pm. If you look at the Trail News at the Oregon City Website they have a list of what is playing. This is right on 7th on the right hand side as you start coming up the hill.
I would be interested to see what other ideas you get. I am always looking for fun things to do with my 19 month old. This summer is totally different than last year.