L.M.
Ok...so many things going on here. First, you were friends once and you both care about your kids so there is some common ground. I agree with the other poster who said that the kids may not have relayed the information exactly as it was said or intended (or maybe they did). I also agree that you can't control what she says but I believe as their mother you can and should intervene when she says things that can be damaging to them or your relationship with them (such as she cares more)...and not because it isn't fair to you (it isn't) but because it isn't fair or good for them. The four of you (or at least you and the two of them) should sit down to discuss this (without the kids there) and explain that you are thrilled she cares so much about the kids and want the four of you to be able to co-parent even if you are no longer friends and that means showing respect to each other as parents (doesn't mean you have to always agree but you do need to show a united front). Ask about the important comments (the ones that effect the kids) and come to an understanding. Then come to an agreement that negative comments towards or about each other should NEVER be done when the children are around (even if you think they are out of earshot they may hear). As mother and father to these kids, by not showing respect for each other the kids were pick up on that. Doing it this way will not be pointing fingers and putting anyone on the defense but rather agreeing to put the kids first.
Second of all, if you did divorce you husband (her ex) you won't get away from her because she is still married to your ex (who is the father of your children).
Good luck.