V.B.
My kids didn't really do that much when they were in the younger grades. But my daughter got her first middle school yearbook this year and she took it to school every day after until the last day and let her friends write in it.
I don't know if my child is "cool" or not, but she also had several of her teachers write in hers.
I think in kindergarten, they don't really know what to do and their writing skills are not that great (so small spaces are hard for them to write in, especially with no lines!)...
I think the most my kids' friends wrote in theirs was maybe they wrote their name. That's about it. LOL