It's funny, the only kid I've needed to talk to lately about my relationship status is the little girl I sit for, who is 6, extremely intelligent and precocious.
However, in the 2 years I've known her, she hasnt even been able to wrap her head around the idea that I am divorced. She claims that all the kids she knows in school have a mommy and a daddy. Even when I said sometimes kids have just a mommy, or a mom and dad that live separately, she rejects the concept.
(She did not reject the idea that kids could have 2 mommies though, and once had a wedding for two of her Barbies.)
I talked to her often of my "best friend" miss pixi, and more recently, my friend Ginger. And now she (and her parents) know that I just moved in with miss pixi. I am sure her parents suspect miss p is more to me than a friend, but no one has ever asked any questions... and I don't talk about Ginger to her parents.
For the record, the dad is Christian and the mom is Hindu, but I do not know if how they practice involves intolerance for queers or other alternative type people.
In your case, if explaining your polyness to their kid makes your relatives question the assumption that Christianity is ipso facto hetero and mono, it's a good thing.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
In a newish relationship with Steve, (34, poly)
"Master," (mono, 34), miss pixi's Dom for 3 years