According to an article written by Rabbi / Sir Immanuel Jakobovits, sending children out during Yizkor is based on the shul's custom, and one should not alter from that custom. He recommends that children and non-orphans read a special "Prayer of Thanksgiving". I haven't been able to locate that reference in that link, but, B"N, I'll hunt a bit more.
I have been to a number of shuls where the rabbi specifically tells non-orphans to stay in shul so that they can recite the communal El Maleh for partisans, Hollocaust victims, 9/11 victims, veterans, Israeli soldiers, etc. I.e. - he feels this is a communal event. (Personally, I think his main motive is to speed the davening and avoid the noise and confusion caused by a huge mass leaving and re-entering the shul. That makes a lot of sense to me.)
It seems that having children and other non-orphans remain in shul is becoming a somewhat popular "trend" in "modern" shuls. Years ago (about 15 - 20) it seemed to be practice in many Conservative shuls. But, I have seen this become a bit more popular in "modern" Orthodox shuls (meaning non yeshivish or Hassidic) within the last 10 years or so. It's far from "mainstream", from what I can tell, but, it seems to be occurring more frequently, now. I can't say how much of this trend is influenced by viewing Yizkor as being a "family experience" (i.e., grandson should say Yizkor from grandpa, for example), is it to have more people recite the "communal" Yizkor, or is it just a decorum issue, as stated above.