Kelly that sounds lovely. For flowers what about a single orchid or stargazer or calla lily for each girl (maybe with a green leaf?) then for you, instead of a massive bouquet maybe one flower representing each child (5, right?) then 2 more for you and Everton. Damn that's getting to be a big bouquet, lol...well you could use small flowers like mini carnations or orange roses...ooh, I like those. That's what I would to to symbolize the blending of your families...but it ain't my wedding, lol. Whatever you decide will be beautiful.
A few hundred dollars is definitely acceptable for the donation, I'm sure he would do it for nothing so really anything is good. The connection you have with the minister is awesome, definitely make the gift personal. If you have time, maybe you can get the kids involved in an art project.