It is different with every case.
The younger the children, the better. The mom really has to put her children before the man, while the children are young (until 18) at least. My mother put men first (it was all she knew).
I think the dynamics can vary greatly. Both parents have to be beyond reproach. But parents are people too. They play favorites. They usually treat the younger children better. It is easier to establish a relationship with them. It is easy for parents to ignore teenagers and their issues.
Here is an very partial explanation of how bad it can be. Each relationship in our family is complicated.
My stepfather really bonded with my youngest brother. He was only 8 years old when my stepdad came into the picture. The rest of us were 12, and 13. My little brother still had to be taken care of. My mother checked out on me and my older brother, when she started dating (after divorcing my dad). My mother actually moved with my stepdad to another city while we were still in high school, so we stayed with my dad (a nut) so we could finish school. They were not married when she moved. So that was pretty much the end of a close relationship with my mother. She had other priorities. If we wanted to see her, we had to go to her. My mother, to this day, has never visited me or my brother in our homes. She never came to one apartment.
My little brother never really "knew" our father (who remarried and had a child).
My stepmother was great, until she had her own child, then she played favorites. I am talking on a major scale.. school, dentistry, clothes, etc. We did without while her child got every lesson, toy, etc.
My stepdad has his own son the same age as my little brother, but he was not as close as the boy was raised in another state (by his own mother).
Now that we are older, my stepdad is pretty good to all of us. The same rules and standards never applied to all of us children equally. His son and my little brother are spoiled rotten and given Hundreds of thousands of dollars, yearly. My stepdad cannot stand his own son but treats him better anyway. (blood thicker than water, I guess). Their children are sent to private school and their wives don't work. The both live in multimillion dollar homes. They both have 2 nice luxury cars and get them replaced every few years. Both have worked or work for my stepdad. Both belong to country clubs (paid for by stepdad). Both have never really accomplished anything on their own and were not worth what they were paid. My stepbrother did finally go to work for someone else and broke away. My little brother drove him off.
Those children will inherit the bulk of their estate (millions). My older brother has a child, and it is not considered so far. My youngest brother lives in the same town in California as they do. Everytime Stepdad has moved so has he (3 states). I am in Georgia and my other brother is still in our hometown in Louisiana. So, I guess out of sight, out of mind applies to my mom and stepdad. My younger brother's 3 girls are their WORLD, to a sickening degree.
The rest of us get nice gifts at Christmas and we were each given a car. We each had help with very modest first homes( and are still in them). We are also kept at bay by my greedy younger brother and his wife. They want it all and want to make sure we are never included in holidays, etc. They run interference with all kinds of crazy drama. My parents are aware of it, have talked about it, and say they don't like it. But they continue to reward bad behaviour. They have never been able to stand my (beyond wicked) sister-in-law. But her kids are GOD.
I am the oldest (48) and have always been the most independent. I had to raise my brothers while my mother divorced, dated and worked. I really did not have a childhood. If I wasn't babysitting, I was cleaning up after them. I guess that is one reason I did not ever want children of my own. I had already done it, and it was no picnic.
I have chosen to stay away from the disfunction. It is all too dramatic. I see my mother every year and a half or so. I have not seen my nieces in 7 years. I have talked to them on the phone when they are at my mother's.
I don't expect much anymore from any of my family. I tried to talk to my little brother last year about all of us moving forward, without re-hashing the past and possibly including the rest of us. He chooses not to. Says his wife has a problem with it. (She hates my parents but still takes all of their money).
I have tried everything I could think of to get along. I give up. My mother is so guilt-ridden, that she is miserable, and she has talked about it. I have always told her that people do the best they can, when they can. She can't seem to move forward and have much of relationship with us now. I am still the adult, consoling my mother the child.
Things I have learned:
Money doesn't make people happy, it makes some of them greedy, bored and miserable.
When you are old, your past actions truly do haunt you. All the money in the world won't fix that.
One party can't fix a disfuntional family, no matter how hard they try. Some people like the disfunction and even create it for their own gain.
You must move on and make your own life, no matter how much you want your own family to be the traditional family. There aren't that many of them anymore. Maybe you can create one for yourself.
Life isn't fair. Nothing About It. But are you going to quit??? Hell No.
My Motto is: Your Friends are the Family You Chose.