FWIW, I think tween might be a little too young for this, but it depends on your kids. The way it worked out for us was:
Bank account (savings) when she had sufficient $ to merit it. For us, this was 13 (bat mitzvah $). Allowance = cash. $ goes into savings but doesn't come out.
At 15, she got a checking account and associated debit card. I was worried, but she had a summer trip without us where she needed to be able to cover lunches, and I didn't want her to carry all the cash. So we transferred enough from savings to checking to cover the expected total + $100, and then we kept an eye on it.
Turns out, she was sort of ridiculously mature and responsible about handling the debit card. So last fall, I set up an automatic transfer for her allowance every month (I think it's 2/3 into savings, 1/3 into checking; she has the ability to transfer over some into checking if she needs it, but the minimum is 1/3 to savings). She then has a part-time job that she can deposit herself into either account. We do all of this via online banking.
She also still periodically works for us to earn more $ (e.g., babysitting). We keep a running tab on a note on the fridge. When she wants $ for something that requires cash or a CC, we subtract it against what we currently owe her on the note, and I give her cash or pay directly with my CC.
My DS (11) just has a savings account. He does much of his purchasing through prepaid gift cards (e.g., PS4 stuff thanks to his birthday stash). If he does want something from somewhere else, I just transfer from his savings account to mine. But he is also not the problem child in this regard (I used to give him $5/wk cash and $5 in his savings account, and he asked me to put it all in savings, because he had all these $5 bills scattered in his room!).