For entry requirements: If connecting in Frankfurt, you can send an email to the border control in Frankfurt airport and ask them what is required for entry into Germany (i.e. transit in Frankfurt) - the email address is on this website: https://www.bundespolizei.de/Web/DE/...furt_node.html

In my case the 'proof of relationship' required is a copy of marriage certificate and copy of spouses passport (if not travelling together), so perhaps you would just need a copy of birth certificates and passports. Having the email from the German border control printed and with me was also useful for a couple of nervous check-in agents and a border control agent in Amsterdam who wasn't quite sure.

For the Covid test: Currently for transit from US to Portugal via Germany you just need to have whatever Portugal requests, which is a PCR test taken no earlier than 72 hours before departure of the last flight (i.e. FRA to Portugal). You will need the results to board the flight from US, so you can work out the timings. Latest details are normally here (as it frequently changes): https://www.iatatravelcentre.com/world.php