I've been visiting the Portland rollercoaster attraction every Summer for years. It's a fun outing, for adults and children alike.
Prices are a bit high but the trick is to find good deals with the hotels of the city. Some even offer half price tickets.
In the Summer though, you might want to avoid the weekends, as the attractions can get really crowded. Waiting in line for more than 30 minutes to an hour is something you'll definitely want to pass on, especially if you make the trip with young kids.
So what I recommend is, if possible, to go during the week.
A piece of friendly advice: the restrooms are pretty gross (be warned!), and some of the people around the park can be very rude, especially on long wait days and when the weather is hot ;)
*This is demo content