welcome to motherhood.
i really think that at 14 they should be fine, of course i don't know them, and who knows, they may not even want to hold the baby this year. at 14 they understand expectations. if one of them are to be the one to hold the baby, then i would simply say, blame it on being a paranoid/protective mom, "your going to have to pay close attention to my baby and take good care of her or you can't hold her." i still use the paranoid mom thing and my boys are 4 and 14 months. a friend/relative will want to take them to wal-mart or something and i say "i know i'm paranoid, but i rather them stay with me or just go to your house another day" i have this fear of someone not watching my kids as close as i do. and it works, some may roll their eyes at me, but at least my kids are safe and taken care of. i am very fond of the saying, i'd rather be safe than sorry.
but i really do think they would be fine. and you will be right there. i would just determine how many pictures they are going to take, and say "ok, you get to hold her for x many of the photos, and you get to hold her x many of times" that way they won't be arguing over the baby during the shoot. but if you don't feel right about it when the time comes, then speak up, it's your baby, noone elses, and you do what you think is best for your baby.
good luck and hope you end up with beautiful pictures.