I would NEVER, ever, ever ever check/gatecheck a carseat. EVER.
They throw those things around and oftentimes seats come out cracked/broken/missing parts, etc. Even if the seat came out "looking" alright, I would NEVER trust one, because you just never know. A tech I know has seen them literally THROWING them out of the plane down onto the concrete tarmac, then putting them in the trolley cart thing to go to the airport terminal. Parents have reported broken foam, cracked plastic, wet/drenched seats, ??!!?? etc....
I would take the seats on the plane with me, if that were allowed.
If not, I would purchase a seat at the destination point, if that is at all an option.
If neither of those is an option, ugh..tough one. If I *had* to take a seat with me, and *had* to check it........I'd probably purchase
2 cheap seats..maybe a scenera for the lil one, and a turbobooster for the older one - and use them just for the trip and then toss them. And pray like heck they don't get damaged and that I don't get in an accident in Germany.