Consider keeping a notebook with new words your child learns. They can write words they learnt at school or words they learn from books or online. There is no set format. You can do whatever works for your child. Younger children may benefit from writing a word and a drawing a picture to show what it is. Older children might be able to write the word within a short sentences that helps them to remember the meaning of word. They could also write a definition they came up with by them selves or found in a dictionary. It can also be useful to write the word in your own language.