D.W.
A rear-facing infant (and the latest recommendations are for them to remain rear-facing till at least age 2) is safest in the middle. Foreword-facing kiddos are safest behind a seat as it offers a level of protection in the event of an accident. Think about it-foreward-facing kiddie in the middle something comes thru the windshield where is it going?
I would put the preschooler behind the passenger seat so you can hand her anything she might need. She will be able to help out with the paci situation as well!