First, you'll have all day to get a feel for how crowded the park is that day. Second, when you pick up your HB meals, ask the CMs (ask more than one) how many packages were booked that day -- average? a lot? fewer than normal? That will give you an idea of how crowded your section might be (remembering that walk ups for HB can happen). Then split up your group. Have 2 adults wait for as long as you think you'll need to get the railing to the far right (next to the tape by the BB section) -- just like with WOC, have the F! viewing area map on your phone so you can show people for last minute review if necessary. Then you can all wait together for part of the time if the kids are up for it or just the two adults can wait and get your spots (since you said you'll do littles in front and biggers in back -- thus not taking up one long row at the rail) while the other two shepherds and kids join a little later. (Make sure the FPs are with the right people so everyone can get into the section!) Your kids are going to love being up close at the rail -- grown ups won't have a bad time either!
90+ minutes is usually only for WOC -- 60 (+ only during busy times or if you really have to have a specific spot) usually does the job for F! But, again, if you know which spot you've got to have, then do what you need to do. Will you regret not having the railing? (Personally, when we've had guests, I've never regretted putting in the time to get the railing for them.)