[Answer]
It seems that there are many people who think that it's necessary to crawl before walking, so it is understandable that mothers would worry if their child does not make an attempt to crawl.

As in the case of the inquiry above, some children sit on their bottoms and push themselves forward using their legs, even though they have learned to roll over and sit up without any problem and have reached the age when crawling begins (10 months or so). They are called shufflers or shuffling babies, which refers to the action of moving forward without lifting their bottom from the floor.

Dr. Peter Robson, a British pediatrician, who noticed that some children move by shuffling in a sitting position and then stand up and start walking without having crawled first, conducted research on the development of these "shuffler babies."

The research found that approximately one in forty children were shufflers who did not crawl and who began walking at an average age of 1 year and 9 months, later than children who crawled. Furthermore, when these children were suspended vertically in a standing position, they bent their legs so that they did not touch the floor and adopted a "sitting on air" position. Another common characteristic was a dislike for belly crawling. Their development was followed until they were older and it was found that these children had no problems with motor ability. Robson concluded that they were "normal children" who showed motor development in a different manner from that of the majority of children.

It is still not known why the children shuffle instead of crawl. The commonly held notion that all babies crawl has now been shown to be false.