Official Title For Witnesses That Aren't In Wedding Party

Hello,
I have been searching the interwebs endlessly for the answer to this question.
My bridal party is completely made of my teenage children. I set it this way and opted out to have a maid of honour & best man so that siblings/cousins didn't feel left out. Me both have big families and we didn't want to hurt anyone's feelings by picking 1 over the other. Having just our children in the wedding party made it simple. The problem is that only 1 of them will be 18 by the time our wedding occurs in 2024, leaving us short 1 witness to sign the marriage certificate.My cousin and her husband are offering the lend us a bunch of stuff they have from their wedding a few years ago, along with helping with decorating and clean up. I would like to give the 2 of them some sort of official title (that's not maid of honour/best man) and have them come up to sign the marriage license without actually being in the wedding party. This way they're honoured for all their hard work. I'm even sitting them at the sibling/parents table that is very close to the head table (with our grandmothers sitting at the head table with us). I feel that giving them a official title and doing the whole proposal thing you see people do for their bridal party will make her feel special and appreciated.