Translation:Although the weather is bad, the children are playing outside the house.
This would have been a great time to introduce tamen. It's easier to think of kvankam and tamen as a pair.
"Kvankam la vetero estas malbona, tamen la infanoj ludas ekster la domo."
It's true that they have similar meanings, but using both together is superfluous and sounds clumsy. One or the other alone communicates the meaning.
Awkward. Although the weather is bad, however, the kids are playing outside. It only sounds right when trying to emphasise something.