The 20-year-old, who arrived at White Hart Lane with Simon Davies early last year, soon took centre stage however, picking out Ferguson to volley home moments after Leonhardsen had levelled.
Rookie striker Kamanan added a third on the stroke of half-time, before Etherington wrapped up proceedings 12 minutes from time.
Spurs: Kelly (Jalal 81), Consorti (O'Donoghue 73), Thatcher, Gardner, Thelwell (Henry 81), Leonhardsen, Piercy, Snee (Barnard 67), Kamanan, Ferguson, Etherington (Slabber 81).
Posh (first half): Connor, Hooper, Jelleyman, shields, Thomas, Edwards, French, Forsyth, McKenzie, Fenn, Farrell.
Second half: Steele, Hooper, Williams, Shields, Joseph, Murray, Semple, Bullard, Green, Clarke, Lehtinen.