Hi Annaboat. We jut returned last week and went to La Fortuna (Los Lagos Resort), Monteverde (El Sapo Dorado), and Manuel Antonio (Tulemar Bungalows).
We loved Los Lagos Resort as it was so close to the volcano, had great pools all with slides, horses on the property with a great ride to the base. The horses were brought down from the meadows for our ride (not tired and hot in a corral) and Alberto our guide showed us howler monkeys, toucans, and an anteater. My 17 yr old loved the ATV tour also on their property and got a science lesson on volcanos and geology as well. Kids are very welcome and safe at Los Lagos and the rooms up the hill are fairly new and extremely comfortable. (Also have satellite TV that helped our kids relax after an active day). Their patios directly view the volcano and your kids would like their croc zoo and butterfly house. We were sorry to leave, but the drive around to Tabacon and Lake Arenal was beautiful and really undeveloped, lined with impatiens, bromeliads, orchids growing wild on trees, and the occasional monkey sighting.
Tulemar was a real treat as well with pillowtop mattresses and octagonal shaped bungalows that felt like treehouses (sleep 6). The windows could open all the way, the bathroom was roomy, and their private beach was safe and fun. They had ocean kayaks free to guests and you could visit other beaches that way as well. Their beach has white faced monkeys and a sloth, loaded with iguanas, and shaded areas all from the overhanging jungle. Our bungalow viewed the ocean from the beds and we were thrilled with our 3 nights there. The staff was great with kids and ours made friends for an improvised bocce ball game on the beach with coconuts. The cost was a bit of a splurge for us but well worth it. We just skipped an extra tour!
You and your children will have a wonderful time. Our (kind of jaded) teens thanked us over and over for taking them on this trip. That was a real bonus reward!