Hi M.! As long as nothing is wrong with that relationship, I would try what I've read. I've had experience with this with my 2 1/2 year old daughter (and I've also read tons about it) and the advice that I received that works is make the "good bye" short and sweet. It really works. My daughter would cry for me when I left but the babysitter would call as soon as I got to my car door 100 feet away and say she is fine and I would hear that. When you linger or feel bad the kids think you have a problem leaving them.
The kids play off your emotions so try (as hard as it is) not to be sad when you leave. Before your ex gets to you or you drop the kids off, tell them how much fun they'll have and ask what have they done there recently. When you get there eplain to them that mommy has to work and you love them and will see them later. Tell them to have fun with daddy. Give them a kiss and run out while you are saying 'bye I love you" in the cheeriest voice you can find. Have your ex call you when they are fine. The first few days will be no change but then it will change dramatically. That is as long as nothing else is wrong (i.e. problems with dad, etc.). You know that best.
Good luck!
S.