Can we check if passport ready? Do we put old # when buying plane tickets?

Its been 3 weeks since we went to the embassy and renewed our kids' passports. Instead of using DX (which I understand texts you when passports are on the way back), I gave them a prepaid RoyalMail envelope. So there's no way of knowing...

2 Qs:
*Is there a way to check how far along in the process their passports are, and when they are expected to be ready? (More than a decade ago, one could call the US embassy, and they did check)

*I need to buy plane tickets. But they require one to enter passport #. Would I just enter their old passport 's for now, when I buy tickets? And then change it later, before we leave?

*If the passports dont arrive a week before we leave (hopefully Aug 1), if I contact the emergency passport phone line, do I have to apply for an emergency passport from scratch, and pay for it?
Thanks

Actually, I just found one answer online! You CAN check the status of your US passport application here:
https://passportstatus.state.gov/

It doesn't matter where you applied for it, or if its a renewal or a child. You just need DOB, last 4 digits of SSN.

It says they mailed it back to the US embassy, so hopefully that means we'll get it soon.

Thanks for the update. 

Actually, I just found one answer online! You CAN check the status of your US passport application here:
https://passportstatus.state.gov/

It doesn't matter where you applied for it, or if its a renewal or a child. You just need DOB, last 4 digits of SSN.

It says they mailed it back to the US embassy, so hopefully that means we'll get it soon.

Good news indeed.

When our son renewed his US passport he also used Royal Mail and when it came it required a signature. He was at work when it was delivered (no notice which day it was going to arrive) so he had to take the delivery note to our local Post Office to sign for it.