My dependents just changed in 2018. This year I have one of my children full time and the other 50% how do I claim? Last year had both 50% and claimed for one.

Only one person can claim the children- Because a child is with you 100% of the time - you claim that child and you are entitled to claim the second child since they are with you 50% of the time. This decision must be made between you  and the other "parent"