You're right, that would be really tacky. LOL! But what you want to say is so key. They don't train their kids, because frankly they don't have time, and therefore they don't enjoy their children.
Imagine for a minute if your daughter were in public school all day, picking up bad attitudes and disrespect for the other kids, while not being disciplined biblically (if at all) by her teachers when she sports these nasty attitudes. Then in the few hours you have with her at night, you're making dinner and preparing for the next day while she does her homework, rushing to give her a bath before bedtime. (This is assuming she's not in any extra-curriculars, in which case you'd have even less time with her.) When could you disciple her? I mean, really. When would there be time? You might be able to faithfully fit in family worship time, but that wouldn't be enough to undo all she learned at school all day.
The reason you enjoy being with your daughter all day is because you've spent so much time with her that you know and understand her very well. When she throws a fit, you know why. You are sympathetic towards her and that draws her heart to yours. And you are training her biblically, which produces an enjoyable child. I haven't always been a full time mom, so I know very well how you really don't enjoy them near as much when you're not with them very much. The longer I've been at home full-time, the more I enjoy my children.
That's what I would say. "It seems backwards, but the truth is: The more time I spend with my daughter, the more I enjoy her." Any more than that, and you'd offend them.
Really, all you can do is be gracious and be a good example of a joyful mother of children. They may begin to ask you questions, in my experience. Then you can encourage them with the truth of God's Word.
Just be compassionate. It's sad that they don't enjoy their children and they really have no idea why you do. And that's sad too. They don't get it.