This is a fun place for kids ias long as you aren't expecting Disney-quality rides!
We took our two small kids to Oaks Park for the very first time the other day - they run a special program during the summer on select days just for preschoolers - $5 unlmited rides for 2 hours and then a storytime session accompanied by cookies and milk.
My kids both loved the carousel and the little train, but heck - you can do that at Clackamas Town Center Mall. They also liked the teacup ride, which was good preparation for Disneyland, and the Frogger. There were several other rides, but maybe my kids (4 and 2) were a little young to be interested. The rides do look a little worn and shabby, but at least they seemed clean.
We also went roller skating, which was the most fun of all and definitely worth the money.
Overall, you should check out Oaks Park if you get a deal on the rides, bring your own food and don't have super high expectations for the quality of the rides.