If Trump was sending them a message, there would be no need to distribute it at the funeral. It could be delivered normally. He already would know what's in it.

So this is probably a message from some cabal member to the Bushes and Hillary. Also, it seemed that Obama and Michelle were laughing, almost joyful -- especially Michelle's expression, while Hillary is reading her note.

We see at first GWB is handing a note to Michelle as they're being seated at the beginning of the funeral, then the rest of the pictures are a few hours later at the end of the funeral, as the casket is being removed.