You've been largely out of your children's lives since your divorce. Perhaps you've been living on the other side of the country or even abroad. Maybe you've been in a recovery center getting help for a substance abuse issue. Some parents don't see their children for years because they're incarcerated.
Whatever the situation, you now are in a position to re-establish a relationship with your children, with shared custody or at least regular visitation. If your co-parent has concerns, they may try to prevent that. They may genuinely have the children's best interests at heart or they may simply still feel angry and resentful over your divorce.
Even if your co-parent is agreeable to letting you back into your kids' lives, it's essential to seek legal assistance. You need to know what your rights and responsibilities are.
Once you get the parental and/or court approval you need to start seeing your kids again, it's important to take things slowly. Depending on how long you were away and what kind of relationship you had with the kids previously, it may be best to limit your contact to short visits at first.
Be especially cognizant of how you talk about your co-parent and other family members to your kids. They likely feel protective of the people who have been caring for them in your absence.
Be prepared to answer your kids' questions about where you've been, why you haven't been around and why you're back now. You may want to seek the advice of a mental health professional who specializes in helping children of divorce to determine how best to explain things. They may benefit from therapy as well.
Don't hesitate to seek whatever kind of help you need. It's best if you and your co-parent can have an amicable relationship so that they can work with you to integrate yourself into the kids' lives without throwing their schedules and routines off. Your co-parent should probably begin to include you in school and extracurricular activities as well as other important aspects of the kids' lives.
If you and your co-parent didn't have a parenting plan because they had sole custody, it's probably best to develop one. A detailed plan can help prevent confusion and conflict over schedules, rules and expectations. This can help make your reintegration into your kids' lives go more smoothly for everyone -- particularly them.