The Heathrow Express is the best way to and from the airport. It costs £21.59 one-way except on Saturday when it costs only £10 one-way.
You can also buy a two-way ticket.
How much should a two-way ticket cost so that it is never more expensive to use it?
Thinking versus computation - it is important to engage your child in conversation about how much a two-way ticket might cost. Should it be twice of £21.90 (mentally, 20p less than £44)? It is usually cheaper than that to encourage passengers to buy two-way tickets.
The key to help your child is to have conversation and discussion. You are not helping very much by simply putting a stack of worksheets in front of the kid.