Ive caught the train a couple of times from Sylvia Park and it wasnt any problem, the lift takes you up, walk across 5 metres and down to the road next to the mall, its at the back, so no motorways to cross
The ticket person will give you a hand with the stroller, just make sure you go into the carriage next to the front of the train, cos they hang out that area I think but Im sure someone else would offer you a hand anyway.
I dont know any malls that dont have wheelchair access, so maybe that mum didnt look properly when at the mall