Hello, and welcome to the forum! And congratulations on the children transporter. I've got four of the former, and one of the latter myself. Or rather, my wife and I has it all together. It also carries a dog from time to time
About your problem: There must be something draining the battery, or the battery is dead and drains itself, so to speak.
No, the Espace's don't usually drain their batteries, but if an electrical failure has happened, of course that could be the problem.
I suggest the following simple procedure to check whether the problem is with the battery or somewhere in the car's electrical system: Disconnect the battery from the car. Then charge the battery fully. Leave the battery disconnected. Next day, reconnect the battery and try to start it. If the car starts without problems, it's probably the car itself draining the battery. You must measure the current drawn from the battery, and you should start pulling fuses until you find the source of the drain.
About your radio. It all depends on what brand and model it is. It could be wiring though, but it's difficult to say. There are no typical Espace-problems related to dead radios, I'm afraid.
Cheers,
Anders