Children in the same family often attend different schools for a variety of reasons, not least the presence of single sex schools in an area. Families have much more to join them together than the schools children go to. Certainly many primary friendships come to an end at the time of transfer, but new ones are quickly made in all types of schools. Selection means school choose children. Most schools choose children.
Popular comprehensives choose on the basis of nearness to school.
The abolition of grammar schools would result in selection into high performing schools being on the basis of ability to pay, or ownership of high value houses. Able children from poorer backgrounds would be the losers.