Love your kids with everything you've got. Let go of your ex, as best you can, and focus on the impact your love is having on your kids. Stay true to that value and you will always choose the high road.
Today, the law serves the best interest of the kids and the mom. Dad is seen as the breadwinner, and thus the "paycheck" for divorcing moms who are showed the divorce brochure and the good deal they are being offered.
She has played all her trump cards and she's still not happy. She's remarried and she's still not happy. She might not ever be happy. That's no longer my problem.
I often pontificate that my ex-wife is angry because she didn't get the happiness she imagined by moving her cheese from one man to a different man. She got everything she wanted in the divorce: the house, the child support payment + insurance for the kids, and the 70/30 parenting schedule, and still she's not happy.
Dad's are just as important as moms. Even with young kids, the loss of either parent is one of the most painful aspects of divorce.