I wouldn't be too concerned as I am sure their Dr is aware and monitoring it. Have they gone for their 18 mos check up yet?

My cousin's baby didn't walk either at that age and wouldn't put weight down. The Dr. started to get a little concerned but wanted to continue to wait and see then she started walking. The screaming would be hard for anyone but I am sure with you ignoring the screams, not carrying them everywhere and the baby missing all the fun, they will learn soon enough.

Having just had a 14.5 mos old start walking I think it was almost better when she couldn't walk as now she is everywhere and always the opposite direction of where I want her! She was easier to keep tabs on when she was a non-walker.