If there are any disagreements, the first thing to check is who has parental responsibility. "Parental responsibility" is about legal status with regard to a child. Whoever has parental responsibility is entitled to make major decisions about a child's upbringing (choice of school, religious upbringing, medical treatment, etc). If more than one person has parental responsibility, then ideally you should discuss what should be done, but most decisions can be validly made by one person without the formal agreement of the other person.
Some decisions must be made jointly, such as taking a child abroad to live (see “What if my ex wants to take our child abroad on holiday, or go abroad to live permanently?”)
If you cannot agree, family mediation or collaborative law might help. If not, then you can apply to the family courts to make a decision.
