Montana in late June can have pretty variable weather. Hopefully it will be sunny and 70s, but with mountain weather you never quite know. We recommend keeping an eye on the weather in the week leading up and plan to pack layers. It's not unprecedented to see a range anywhere from 40s and sleeting to 90s and pure sunshine.
Yes! You're welcome to bring your kids! Please be advised, however, that the wedding venue, Free Cycles, is a working bike shop and there will be hazards for children in both the indoor and outdoor areas of the venue.