Our youngest was 4 and had his birthday on our trip, turning 5. He is very small for his age. The tour companies are great about accommodating kids. He ziplined with a guide & absolutely loved it. We did a tubing trip at the Springs hotel and they took him in an inflatable kayak with the guides because he was nervous to be on his own tube. The only thing he couldn't do was the waterfall rapelling. Not because of his age, but because he had a feeding tube in his stomach and they were afraid the harness would catch on it. They gave us a guide to go through the jungle with and my husband and I just switched off on each of the rappels. Overall, they are so great about dealing with young kids.
I think the only issue you might have is river rafting. I know a lot of the rafting is pretty challenging, so much of it has an age limit of 12 I think. However, there is so much for kids to do in CR that I wouldn't let that deter you from taking a younger child. My only hesitation would be if they are not adventurous kids. My kids seem to have no fear so they love adventure trips. (unlike me, who has a lot of fears!)