Congratulations A.!:)
Your concerns for their feelings are very normal. If you can, take them with you to appointments so they can see what the doctor is telling you. Allow them to hear the babies heartbeats. That will bring the reality of their siblings to light. That might get them excited. Maybe take one child, alternating each month would be an adventure for them.
After the babies are born, I would continue to have the kids involved such as, tossing the diaper into the trash.....helping feeding (with you or your hubby's assistance), sharing the alphabet with the babies, reading books (short 5 minute kiddy stories and rhymes), anything simple and fun. Us adults sometimes forget that kids need a lot of things explained to them. Even if it's simple.lol. I've done it.
Good luck with everything!
M. :)