Hi going to London this weekend (thanks to all the help people have given me before!) I will arrive at Paddington at around 530 on Friday then need to get to St Paul's. The next day I need to go to Victoria and back to St Paul's, then the next day from St Paul's to science museum, possibly central lòndon/ Covent Garden. Then back to Paddington. I understand its better to just use contactless on the tube and it's limited per day. Is that right? Would buying actual journey tickets to where I want to go be better?
I think you can use contactless as many times a day as you like. The number of times you use your card is added up and after a certain amount is reached the card is Capped and you don't pay more.
>>> Would buying actual journey tickets to where I want to go be better?
Absolutely NOT! All of your journeys are within the central zone, so you'll pay £2.50 for each one using a contactless debit card. (As you say, your total fares in a day are capped too but that wouldn't seem to be irrelevant here anyway). If you go to a ticket office (or machine) to buy a ticket, you'll be charged a staggering £6.30 for each journey! Use your debit card!
// Oyster cards would have been better I think with kids. //
unless said kid has their own contactless card (or is under 11), it's pretty much the only option.
https://visitorshop.tfl.gov.uk/en/london-visitor-oyster-card and then (before your first journey) ask one of the staff in any tube station entrance hall to add a "young visitor discount" to the card.