Don't waste your money on a rear facing pram - we had the same problem and went and bought a rear facing pram but DD hated that too. We now have a pram in the cupboard that has only been used once! I posted a similar thread a few months ago asking for suggestions but nothing really worked. DD just suddenly started liking the pram at about 5 months old which a lot of the replies to my thread reported the same thing at around 5 months.

Sorry Anna you seem to have the reverse problem! Atleast your DD enjoys the sling. Hopefully it's just a stage and both your bubs will start liking the pram soon.