I have a trip planned to see family with my children but trip was called off by ex in the last few days before trip. Does she have this right.

Who has custody? If she does, was this trip to be on your visitation time and did not interfere with her time at all.

I planned a trip to Florida from Missouri with my kids to see my family who lives there. I had the OK from the ex months ago and planned family reunion with pics and cousins coming home from college to see my children. This is the week following Xmas weekend. My ex decided that she didn't want me to take them. We have 50-50 custody but it is her week. She already said it was no problem but is now backing out. What are my rights?

None at all, it is her week and completely up to her.
In future while still not binding, get her to agree in writing.
How old are the kids? Have you and her been having trouble, is this a way to use the kids against you ?
And why was the trip not planned when you had them ?

I assume you made reservations that are now non refundable. Do you have any proof that she agreed to let you take them on her time? If so, you can sue her for the lost funds that resulted from her change of mind, but you can't force her allow it.

You could try asking for an emergency ruling from the court that issued the visitation order and see if they will order it, but I wouldn't hold much hope on that.

How old are the children? Does she understand how this will affect the children's attitude towards her if she takes this trip away from them?

Did you offer any trades or compromise as far as the extra time you had wanted with the children ?