I wrote "the girls are walking by her side" and was marked wrong. The phrase I used is the same as "walking next to her." I reported it.
Morfix explains: ליד means "by, near, at, beside, next to" https://www.morfix.co.il/%D7%9C%D7%99%D7%93
"By one's side" and "beside one" are pretty much the same thing. In this case, "by her side" and "beside her."
Many thanks in advance, Duo Hebrew Team!