We're not sure if ds (6) or dd (11) will be brave enough to try some of the rides like space mountain although they will meet the height requirements. I'd love to hear how you handled riding when kids didn't want too!
All 5 of our children love rides and have always been willing to try anything,(although, the 2 little guys are afraid of the Haunted Mansion

).
As long as they met the height requirements... we just took them on with us. (Space Mt., Rock -n- Roller coaster and Mission space were the exceptions, because
I put my foot down and made them wait another year).
We tell the kids, if something makes you nervous or frightened, close your eyes and hang on... it will be over in a couple of
minutes.
Do what we've done...
Get to these rides early, and grab fast-passes. Meanwhile, have one parent ride, in the stand-by line, first, and see if you think the kids can handle it. If so, comeback for the fast-pass time. If not, then the other parent can ride, quickly at the fast-pass time.
If you are certain one or both of the children won't ride, you can, either...
1) All of you wait in line and do a kid swap... prior to boarding, tell the CM you are doing a swap. One parent will board the ride, while the other waits with the kids and when that parent returns, the other parent boards the ride, reducing wait times.
2) While one parent rides, the other takes the kids on a different, nearby attraction. Pick a place to meet, and then switch. We usually do this, so the kids aren't standing in line for an attraction they won't be doing. Example... the Buzz ride is near Space Mt., Beauty and the Beast Show is near, both Tower of Terror and Rock & roller coaster.
3) Or, while waiting for someone to ride, grab ice cream, a shaded bench and relax, or hit the gift shops!