I have been to Tamarindo and Arenal, and I would go back to both with kids. Tamarindo is a beach town. You could hang out at the beach, fish, snorkel, do a day trip on a boat. Arenal probably has more to offer kids. You could probably do one of the easy rafting trips, and definitely hike and see the volcano. There are plenty of kid friendly resorts there.
Costa Rica would be great with kids- I can't wait to take ours! When I went with DH a few years ago we went to Arenal; there are tons of family-friendly tour companies there. We did rappelling but there are lots of options for rafting and zip lines. Nice hot springs in that area too.
We also went to Tortugero, way on the north east coast. I wouldn't want to get there on my own (rough roads & a boat ride to reach it) but we booked an all-inclusive option through Turtle Beach Lodge that included RT transportation from San Jose, meals and tours while we were there. We went out on the included boat tours several times a day & got to see tons of wildlife. I kept thinking how cool it will be to come back with kids & even now DD loves seeing pictures of the birds or monkeys there throwing fruit at us from the trees. We did pay extra to go out on a night-time beach trip to see turtles laying their eggs; such a cool experience & it was worth the long trip from San Jose to do.
FWIW, I felt like our budget went further in CR than on most of our other trips. Hotels were all less than I expected when I first started planning, food was cheap and so good, and the people there were really friendly & hospitable.
We went a few years back to arenal and Manuel Antonio. We can't wait to take dd in a few years! We did a great tour of the cano negro out of arenal and saw a crazy amount of wildlife. We also did a nice tour of the park in Manuel Antonio. The only thing that would give me pause about am is how rough the ocean was at the beach we visited but I've read that they aren't all like that. We really loved costa rica.