Have you tried implementing sign language into your classroom? I do this daily and the infants really pick up on it. Other than that the best way to increase their receptive language is to talk constantly to them and explain everything you’re doing. Like for example, when you’re taking them for diaper changes. Talk to them during it and say things like “ Can you point to your nose? Can you show me your eyes? I like your shirt! Can you show me the truck on your shirt?”