I like the "we love more than one person" line with little ones-because they inherently understand that since they love more than one person & sex isnt part of their repertoire yet.
But the "this is my gf" line works well also.
We have 4 kids, a precocious and inquisitive grandson, 10 neices/nephews & 2 kids who regularly visit.
I have always let their question lead the depth of my answer, but either of the above mentioned starting points works great ( I have used both ).
"Love As Thou Wilt"