Who has right to claim children as dependents

Hello, I am wondering who has the right to claim my two children as dependents in my particular situation. I have 2 children and have been a single mom for 4 years now. Since 2018 filing year, my ex and I have each claimed one child. However I am wondering if I have the right to claim them both because they are with me much more (80%) of the time, and I provide more for their financial needs. 
I appreciate any insight into this. Thanks!