JUST DONT.
Many of their families are conservative, but the little girl’s friends are in GS so she joined. And if these families (the adults) do support things you do not, remember that they are Americans and have the right to believe what they believe.
You not buying is fine. Don’t tell them why. Everyone should have the right to peaceably assemble and legally sell too. Golden rule. And these are children; don’t politicize what to them is just selling yummy treats.
Girl Scout Alternatives
https://wehavekids.com/youth-programs/Girl-Scout-Alternatives