Boulder cave is about 45 minutes from Yakima making it a very accessible activity. It is also higher in elevation making it cooler during the summer. There is a nice parking lot ($8 a car I think) with bathrooms. The trail is wide and smooth although somewhat steep for those of us not in shape. There are many viewpoints and great photo opportunities. You WILL see lots of wildlife along the trail.
The cave itself is very easy to walk through. They have made the path flat and wide. There are many nooks and crannies to climb up into that are completely safe for kids so they can really have fun without the parents worrying too much. There is also a stream running through the cave which is fun to cross by hopping from rock to rock. There area few nooks on the other side of the stream to explore but not much. The entire cave is not very long (think short tunnel) and you could almost follow the main trail without a flashlight although I don't recommend this.
Behind the cave are a bunch of large boulders and fallen trees that you can climb over to reach "Devil Falls" which is a small waterfall and pool of water. Most of the time you will find families taking pictures of their kids but once I found teenagers in their underwear having a good time (oh to be young). You can climb behind the falls if you are careful and there are multiple ways to go through the boulders and back to the cave so you can spend some real time playing here.
If you are really adventurous there are two other option. One you could continue on up the hill from the cave to other destinations. Two, you can actually walk the stream path all the way back down the hill to your car. This is an adventure for the young and healthy.
Overall a great experience and I've been there many times with friends and family.