Is it okay for me to tell my kids that my ex is buying their affection?
How is your ex buying your children's affection?
My kids only love her, not me. They want her to come around more.
It's doubtful that they only love your ex, and not you. You should be grateful your ex maintains a relationship with them. Interfering with that is unfair and will likely cause your kids to resent you.
That's not true, I love my kids, so they will love me.
Well I am glad you care for your kids. You and your ex seem to care a lot about them and you should show them how affectionate you both can be around them. Include her along side some activities with your kids and you can all bond together.