Before getting too PA/anxious about it, I'd try having a word with them each individually. Give it a week, and if things aren't changing, try a note.
I think something like,
"Hi [neighbour's name], As you know, we've moved into the flat nearest the entrance door and have been here for a few weeks now. It's been great to meet the neighbours and we're really keen to meet people and settle in. Just a small point, but the prams left outside our front door make it hard for me to get in with shopping/get our own pram in/something else, please could you fold up your prams and take them to your own flat? I realise this is a change because our flat had been empty a while, but it's becoming a real problem for me. Hope you understand and can help make it easier for us all using the communal entrance.
Thanks, Possum 18"
If they don't comply within a few days, I'd try knocking on a few times to talk first and then if no luck another note saying something like,
"Hi, I've tried to talk to you a few times about the issues caused by prams in the communal hallway. I'm really sorry we've not been able to resolve it, but I think I don't really have any other option now than to speak to the management agent. I'd be very keen to avoid this, if you could stop doing it then I won't write to the agent until xxxx date. Again, I'm really doing everything I can, but this is making access in and out of our flat really problematic. etc"
If they don't respond to this, then you've every right to call in the agent/fire brigade, but give people a chance first. What an irritating situation.