All the players have to want the same triad for it to fly right. Are all the players wanting this configuration of open relationship model?
Do you all agree to the personal standard you create together for this polyship? (If it could help, here's my personal standard.
Or is the more natural shape "it just wants to be" is a "V" with him as the Shared Sweetie, and her and you as friends?
Or something else?
<--good secondary area
The kids and living in a small town seem to be concerns -- do they share in same? What agreements could to make to address the concerns?
Talk to them. See where each person has their wants, needs, and limits at and what needs to happen so ALL players can feel good in the polyship there.