Belize for beach and rainforest experience?

I was wanting to go to Costa Rica in August/early Sept but can't find a flight so am looking at some other options. Basically want somewhere where I can get some beach time (gentle beach as will have a baby with me) and also see some animals in the rainforest. Does Belize offer this and what will the weather be like then? Where is a good (and safe) place to go?