You could get your child to learn a list of difficult words*. Please don't!
*actually, there are ways to do this, and it's coming soon...
How to teach more Complex Vocabulary:
It's best if your child encounters new words in context - in something they're reading for pleasure or information - and you talk about what the word means. That way they care, will remember and are likely to use it in future.
TIP: If you do learn a new word: ask them about it again, later - to see if they remember.
If your child will let you, sit with them for five or ten minutes while they're reading - whether that's Wikipedia, a magazine or fiction - preferably something they've chosen themselves. If they're not keen, try 'helping them' with an internet research homework: where your child will have to read a lot of online text, some of it tricky.
It's unlikely your child will ask you what a word means directly. Kids are expert in 'guessing' and also in hiding their ignorance (as are adults).
So let's assume they'll let you sit with them.
Silently read the text at the same time and try to pick out which words are difficult. This may be because you don't know the meaning exactly yourself! Let your child know that you don't know either and they'll love you for it. Say 'shall we find out what it means?'
Ask 'what do you think [tricky looking word] means?' DON'T ASK 'are there any words there that you don't know?'
If they do know what it means - say 'great'.
If they're close to knowing but a bit off - say 'shall we look it up?' Let them find an online dictionary, or type 'what does [tricky word] mean?' This teaches them how to do it for themselves.
If you can, talk to them about ways the word could be used. Explain connotations and formality, whether the word is negative or positive, common or rare and if it is used differently in different contexts.
If you're feeling particularly pushy, you can also give your child a bookmark and ask them to collect 'new words' on it for you to talk about together later. This can be a competition with siblings to find the most or the most interesting words. Talk to them about tricky words little but often to get them into the habit.
Now they understand tricky words, all you need to do is get them to put them in their writing.