This is the note.
My brother (his name is Timmy, by the way) has deduced that it was written with a black marker pen. We don’t have any black marker pens in our house, just biros and crusty paint brushes. Therefore, the kidnappers must have brought the pen with them.
Tomorrow we’re going to go around every shop in town that sells black marker pens and investigate. Timmy really wants to go tonight, and I would take him, but it’s Granddad Maloney’s (or Maloney Pony as our little sister Clara aka Squelchy Froglet calls him) one hundred and fifth, which means custard surprise night.
