score:1

Accepted answer
string _foreignkeyname = "children"
guid existingchildid = "{some existing guid}"

var parent = _context.set<parent>()
                     .include(_foreignkeyname)
                     .wheredynamic(x => "x.children")
                     .where(y => y.id == existingchildid).any()) 
                     .firstasync();

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