Using the children is also, I think, to instill guilt in their downline and potential recruits .... "Look at my child, aren't they wonderful, don't they just have the best life?..... if you work hard you won't let your children down like you do now ."
On a safety aspect why oh why do they always show them in clearly identifiable school uniform? (OK, I realise that it's usually to show off that they can afford private school but FFS, it's one of the basics of internet safety where children are concerned)