Gift Question???????????????????/

Ok, so the three girls I babysit are bringing up the gifts at my wedding (I've been babysitting them for 10 years, so I really want to get them something special) The oldest is in 10th grade, the next 9th, and the youngest, 6th. They really have everything in the world, so I was just wondering if you had any ideas on what to get them!? I've already bought them matching clutches to go with my bridesmaids. And the oldest girl turns 16 the month before my wedding and I'm getting her a initial necklace from Tiffany's.

They are however, wearing the same dress as each other (I really wanted them to be in my wedding party, but I have 11 bridesmaids and didn't want to get insanely overboard - not that I didn't already haha) should I buy their dresses? or any ideas on what to get them??? TIA!!