My nieces are 13 and 14, my nephews are 4 and a few months. I intend to keep buying them presents indefinitely. Certainly until they are independent adults. My brother and sister get a small birthday present but we only buy for the kids at christmas, this is by mutual agreement.

ETA For christmas could you suggest a family KK? So, if you have 3 kids you pick three names out of a hat and buy three gifts, if you have two kids you pick two names etc. Seven is an awful lot to buy for christmas, although I would still buy them each a birthday present.