You want the UK passport numbers, because that shows they're UK Citizens and don't need visas.
If you need to correct the form just cross out what you put and write the corrected amount in.
I wouldn't stress over 2 days, but be as close as you can with your dates.
Other than finding an old paystub or the actual NI card there's no other way to get the NI Number other than to send in the form. Just put yes there is a number, but it's unknown...shouldn't be an issue.
