London public transport with children

[LesserKnownKardashian]

Can any Londoners please help me as the tfl website is just confusing me more.
I want to travel by underground from zone 4 to zone 1 and back. DD is 13 and has an oyster zip card which has never been used. Can I top this up at the tube station? DS is 8 and therefore doesn't need an oyster card, right? And I just use contactless?
How do I work out how much to put on the oyster zip card? It will only be used a couple of times a year so I don't want to put too much on.
TIA

The 8yo travels free with an adult. Just tap through the big gates with them. Top up the Zip card for the 13yo online (though it’s a dreadful website, takes ages to get in and then very hard to navigate) or at a machine at the station.

A fiver or a tenner will be fine. You should be able to top up at a machine at the station.

I think the daily cap on the teenage oyster is £1.60. We did two tubes, two buses and two zone6-zone 1 train journeys for this last week.

Putting - maybe that’s a weekend rate? My dd commutes to school and it’s £1.80 a day (90p each way).

You'll be charged 80p off-peak or 90p peak each way for the teen's zip card.

A contactless card will be ok for you.

Your 8-year-old will travel for free with you on the TFL network which includes the tube and buses. (If you go on national rail they will either need a return ticket or their own oyster card)

Enjoy your day!

Thanks everyone!

Just to add - you and the 8 year old need to look for the bigger “family” gates to go through (the ones with a picture of buggy/suitcase on them) - the small single person ones don’t open long enough to let 2 people through. 13 yr old can also use this gate but needs to wait for it to close and then tap separately.