D.S.
This is definitely an anxiety dream. This trip will probably be much harder on you than it is on your children and husband. They will have fun spending a few days with a different dynamic than it is when all of your are together (nothing against M., it's just a different atmosphere when it's just Dad). So, keep in mind that it's YOU who is anxious, not them, when the worried creep in.
One way to look at this dream is that your husband managed to keep the children safe during that dreamed disaster, even though there was terrible loss. If you were home during a real fire, would that really make anything better? So, expect that whatever could happen in your absence, your husband will handle it well and keep your children safe, even in a catastrophic situation like the one in your dream.
Then, pray pray pray!! It's important for you to be with the rest of your family for your sister's first communion. God will honor that. He wants you to depend upon Him, and this is a perfect time for doing so. Pray Psalm 121:7 for your husband and children, "The Lord will keep you from all harm--He will watch over your life." All of Psalm 121 is a good protection passage. Praying to have peace of mind would be good, too. Listen carefully to the First Communion ceremony and remember that it still applies to you, too!
Blessings to you!