In case of a friendly divorce, should the children (say 8 year old) be able to choose with which parent to mainly live or should this decision mainly rest with the parents?
The short answer is no. (This is my opinion, I am not a lawyer.)
Joe, I think a large part of this has to be the legal part. Where I live, custody is mediated or awarded by the court system. At eight, a child will be heard in that process but the decision lies with the best interest of the child, not in what the child wants.
If you have joint custody, I would highly recommend that both parents live close enough to the school so that school is always the same. Set up a bedroom in both homes and try to keep the same rules and routines in both.
In our situation (we divorced when our daughter was 8), we each have her for one week plus a weekend. No decisions are made by only one parent unless it was an emergency. We 'spot' each other for special events and holidays. We totally parent and cooperate as the family we are. I recently have had a medical issue, so my daughter has been at my ex's for a month. Flexibility is key.
If you do not have joint custody -- that pretty much means your decision has been made. If one parent moves away -- they are the one that loses access. The only reason is to allow the child to stay in the familiar environment. I say this 'blind'. If both parents and the child all agree the child should move -- then that works, if the court agrees.
You say this is friendly. I hope so because our situation has added a good something to our lives. We've both repartnered with wonderful people and our daughter loves them.
What the child wants should definitely be worthy of consideration but the fact remains that rarely would what the child wants actually be in the best interest of the child.
There could very easily be one parent that was always the 'nice' parent while the other one was always the one that maintained discipline.
It would be natural that the child would want to live with the 'nice' parent but having the child live in a house devoid of discipline is not entirely good.
Another thing you should consider is that even if your divorce is amicable that does not mean you can decide on the child's future without any state intervention.
At the very least there will have to be a social worker who has to work on the case.
Also do not agree to anything without the consultation of a family lawyer. Your will most assuredly regret that. I would also not contact your ex in any manner without at the very least your own lawyer present. There has to be a record of all communications.
It is possible that even if things seem good now it can turn sour at any time. Remember that your ex can get her own lawyer at any time and if you do not have your own representation then things will go bad for you.