Series 1, episode 2, about 13 minutes in, also 22 mins in, boy in beekeeper costume clearly has a retainer, there's several shots of it as he talks.
Series 1 episode 3, about 13 mins... same boy is putting his retainer back into his mouth. 3 mins later at 16 mins, he lends Wednesday his retainer so she can use it to pick a lock!