Poppy,
The first decision you have to make is what part of Costa Rica you want to visit. As you probably know, there are two major airports; one in San Jose (central part of the country) and one in Liberia (Guanacaste northwest part of the country). Make that decision and the rest gets easier. You didn’t say how long you plan to stay or what “roam” means to you. Length of stay will affect the amount of roaming you can do. The summer “green” season is a good time to go and rates for accommodations, rental cars, etc. tend to be lower at the time of year.
My wife and I have been to CR about a dozen times since the 80s. We are returning this coming Christmas and being joined by some first time visiting friends from Toronto. We have seen much of the country except for the deep south. Roads and access to the deep south are difficult and that area receives the most rain during the green season. Roads can be washed out or impassible. We have only visited the Caribbean coastal area (Limon) once. It is not the safest area to go and it has the least accommodations. Tap water is also not safe to drink there but it is in most of the rest of the country.
We have now settled on flying in and out of Liberia (LIR - Daniel Oduber Quirós International Airport). We typically spend a week in the Volcan Arenal area in a rent house. That part of the country is an active vacation area – ziplines, white water rafting, waterfall rappelling, etc. But it also has some amazing hot springs resorts, eco-zoos, etc. for the younger people.
We then usually spend a week on the Pacific coast. Our choice of areas is Playa Hermosa but Playas del Coco and other nearby beaches can be good too. Our choice is to stay fairly near the airport in Liberia (less than an hour away). We sometimes rent a house, at other times, a small local hotel.
The Manuel Antonio National Park area is also a big tourist area near the town of Quepos. It is more accessible from the San Jose airport.
We rent a 4 wheel drive car from Vamos Rent-A-Car https://vamosrentacar.com/. There are other rental car companies near the airport too.
Our experience with the seasons is that during the wet season (summer) rain rarely ruins a day. Rain typically falls for an hour or so in the afternoon or in the evenings. Depending on where you are in Costa Rica, the temperatures can be in the low 90s which is definitely warm so accommodations with air conditioning are a must. In the mountain areas, spring type weather prevails.
Like almost every country, the smaller the town, the less likely people are to speak English. However Google Translate on your phone works wonders. Everywhere you go, there are little restaurants and sodas (small lunch counters sometimes with limited seating). The food is basic and fresh. So far, we have not experienced any issues with food as we do in Mexico from time to time.
After you have done some research and have an idea of where you want to go or what you want to see and you would like some recommendations, feel free to PM me. I may be able to assist. BTW: Our son’s first trip to CR was at about your child’s age. We also took our youngest daughter to CR when she was 6 years old.