My daughter was a bum scooter and it was the first time I had seen a child use that as their exclusive way of mobility compared to occasionally to get to a toy instead of standing up and sitting down again. Sure took it's toll on her pants.

I am finding a lot of children are walking later and later in general these days with 18 months not that unusual. The fact she isn't even pulling to a stand is a bit of a concern.

I am guessing the parents may be giving in to the screaming and the child has learned to scream and wait for help rather than doing it herself so a vicious circle of dependence has been created.

At the same time working on leg strength through games can be fun - laying child on her back and moving her legs like bicycle riding. Also sometimes it isn't the legs that is the issue but arm and trunk strength so make sure she is on the floor and not in a contraption of some sort so that she needs to use her trunk muscles to support sitting instead of lounging in a seat. Do arm games with waving things over her head. Also watch her eye movement and hand/eye coordination. A child that can't see properly is too scared to walk. She may look like she can see but the world could still be very fuzzy. That is one of my daughter's issues although she refused to wear the glasses prescribed. When younger she bumped into things a lot because her depth perception was dreadful but now she more or less has it figured out. Cut corners short still and ends up with bruises but rarely falls anymore.

I guess my point is that this could all be very normal. On the other hand it could be an indicator of developmental issues. I would observe the child as much as possible looking for skills she can do and what she gravitates towards as well as what she avoids and that might help to figure out what to do to help her along.