Tea made an appearance in the Series 1 episode "Horrid Henry Goes to Work". Bill tricked Henry into spiking the tea with ketchup and mustard.
It was also one of the ingredients used in the All-in-One Breakfast in "Horrid Henry, Time Manager".
It is also the term characters use to refer to a meal in the afternoon, supposedly supper. Mum is the one who makes it the most, though Dad sometimes cooks as well, as seen in Horrid Henry and the Dinner Guests." Mum sometimes punishes Henry by not allowing him to have tea, saying he can have it the following morning.