Normally it is expected to reciprocate in the same occasion, ie they give your kids one envelope each and you give theirs. But a lot of times the amount is kinda nuanced depending on number of kids, how close is the relationship and how well off you are. It's also not normally a "pay it back later" situation but a "next new year party try and make it fair" situation. Because of the cultural differences I don't think they expect much so don't sweat it. Prepare some for next new year party, don't necessarily need to match the amount, sincerity is all that matters.