How does oyster card work with child fare (example)

This question is related to...

Oyster Card - how does it actually work?

But it's not the same.

So basically I'm traveling to London as a tourist and plan to stay for 1 day seeing the sights, etc. - 2 adults and 2 children, one aged 5 and the other 13.

The way I understand the Oyster card is, I get to London. I plan to park in North Ealing car park which is in zone 3. Now I need an Oyster card for each of us to travel.

So I buy 2 for the adults, this will cost me £10 and I'll put on £7.70 credit for each of us (the "Cap" is £7.70 for zone 3.) for a total (so far) of £25.40

I'll also need one for the older child. The Cap for an 11-15 year old is £3.85. Now the website states that he will need a "Oyster Zip Photo" card or something? Can this be bought at the station just like the adult ones? Does it also have the £5 refundable deposit? And if not, do I need a full (adult) price Oyster card for him too, so another £5 + £7.70 credit?

Then I have another child aged 5. From my understanding, she can travel for free because she's accompanied by an adult who is using a "Pay as you go" Oyster card?

Then to get my refund, can I just go into the same station where I bought them and hand them back and get my £15 back?