Costa Rica is a kid-friendly country, or at least the Ticos are very kid-friendly people.
Flying from the United States is easy but from Europe there are often direct one way (11 hours and 6/7 hr timezone shift from Europe) but not always the other. The option of traversing via Miami is pretty dire - the ESTA visa process adds 60$ pp, entry even for a transfer is very heavy duty with finger printing and double queueing and agressive - nb also the exit fee from CR of 30£ pp isnt always in the flight cost. If you can traverse back other than through the USA do.
Working out and researching the timing is important. Turtle lay on the caribbean side Aug for example when your guide may be stumbling across them but at other times they will not be. Last week Aug the tourists disappear ( you may get some discount but probably hotel maintenance sounds aswell) but summer is otherwise busy. There are cold peroids and the rain is worth understanding - it can be very heavy.
The pluses are - its pretty safe, the roads arent bad but occassional tracks mean it is worth getting a suitable vehicle and sat nav if you want to drive (you will probably get value from it, although big city traffic can be dire), the food available contains much for fussy eaters - its not often great but it not just spicy dishes, there are well organised activities like white water rafting, canyoning/rapelling, zip lining and paragliding (they are not cheap! and the hotel organising them can add up to 25% so use the internet to keep an eye on costs) plus a fair amount of wildlife (turtles sloths monkeys) and generally good guides (although they can be a bit overwhelming for kids by the third time.). There are some pleasant beaches ( part of the coast is nature reserve for turtles and not accessible) and the water is warm with good body boarding/surfing waves on the pacific - be aware of rip tides (see separate article on water safety) and don't rely on lifeguards (if present at all) paying good attention. There is good internet in lots of places but if your kids are addicted you will need the right type of adaptor (not a european one) and many hotels have a communal computer - with USB points you can charge from. The weather is generally pretty warm and the rain can be an experience in itself as well as an issue - be careful with swimming pools when the often spectacular lightning is around. Generally most places will accept dollars and many are scrupulous in giving you the midmarket rate i.e. better than any bank BUT some hotels and restauarants will gouge you for an extra 10% by rounding down to 500 C. If you are worried about the heat there are many upmarket places with aircon. One of the nice side effects of kid-friendliness is that there are often substantial discounts available for children, for example for park entrance fees, tourist tours and transportation (but not the public buses, it seems). However, many times the discounts are not advertised at all, so you have to ask specifically and often they are for under 12.
The negatives were, the Turtles laying is an amazing sight but for children there isnt the spectacular mammal life of an African Safari (but if you have a broader wildlife enthusiasm there is lots more), the guides are good but children can tire of the detail and if you are not using one they can be aggressive especially if they feel you are following them - hard not to at times as many of the reserves are actually quite small former agriculural areas, the food is often very pricey for the quality, made much worse if you choose somewhere you are a captive to their location e.g. eco lodge out of town or Manual Antonio (this is one time a car pays off) and the beaches arent quite Caribbean or Malay - but the pacific is warm and good for surfing. There are some good top end hotels and they tend to charge pretty close to US prices but the mid market and budget harder sometimes to assess from afar. Roads are not particularly kid-friendly in CR. Traffic moves fast and sidewalks are often absent.
Speaking to a number of people gave a range of responses from disappointingly underwhelming and expensive (one american traveller commented ruefully "Yanks have ruined the place" - referring to the pricing) to loving the great activities ("the best ziplining ever" and "brilliantly organised white water rafting" were two very valid comments made) and the wildlife and the Oceans. Best guess if that if you have already done amazing wildlife safaris and beach holidays or are looking to travel cheap by choosing a less developed country you may be disappointed but if you are looking for something which encompasses a bit of each plus some great activities, and are prepared to pay near United Staes or European prices for it there's lots to enjoy.