Hmm, you could tie in an internal carseat seatbelt sensor that sends a signal that causes a keyfob attachment to go off if the keyfob moves out of range of the seat without the all-clear (unbuckled) signal being sent. As well as a low-battery reminding chirp, like a fire alarm.
Practically, though, to reduce (can't eliminate, but it would reduce I think) the chance of leaving your child, make a habit of tossing something into the carseat after removing your child, something that you can carry on your person normally and is visible. Say, a bracelet with their name or something? Then if you get to work and see the bracelet on you, you know to go get your child.
IDK... I don't have to bring my child to the sitter on a regular basis so I have no pre-established habit, but if I did, and the "child delivered to caregiver" circuit was triggered while Jed was napping... :shudder: