Child custody matters whether you and the other parent are married or not, can be devastating and emotionally draining. You need an advocate to walk with you and guide you. Child custody matters that cannot be agreed upon are decided by the Judge. Make sure that you have the best opportunity to advocate for your children and their relationship with both their parents, by having an attorney walking with you.
While child custody decisions can be modified, after a Judgment it requires a showing of a change in circumstances. That is a higher bar than one parent wanting to change the schedule. It becomes more challenging after an arrangement is established too, because everyone, most importantly the children, get used to the schedule the way it is.