My dd is 7 and we use nimbl.
She gets £5 so week and is expected to save some - it has a feature to move money to a saving pot.
I do not buy magazines, pocket money toys, sweets etc - she is now expected to buy these herself. I was probably spending more than £5 on these items due to pester power. She can do simple subtraction of her spend mentally and back that up with the app. We talk about how many weeks she would need to save in order to purchase certain things - she loves Claires which is very £££ and it makes her think about the cost.
We decided to go down the card route
A - because I never have cash and would always being IOU - ing her pocket money.
B we live in a world where digital payments / banking is the norm. She will need to learn these skills of mentally keeping a tally of her money and checking her banking app. Better to start early.
She can now use a self serve checkout and pay by herself or pay at a counter - aldo getting confidence and independence. (I am usually nearby and have discussed/allowed purchase or Dh has)
It is a shame that we have to pay for the card but it is cheaper than Go Henry and so far for Skills learnt/ discussions etc.has been valuable.