1) Be a good dad, as advised above. Your children need to feel safe and secure with both parents. Be careful, if you push holidays etc then they will feel compromised in your company. Especially is this the case if your ex is strong in the truth. She will train them to have a bible-trained conscience and your children will feel very bad if you push anything that conflicts with watchtower ideology.
2) Be interested in their theocratic progress. Ask them how they’re getting on at meetings, giving talks and if they have had any ministry experiences. You can still casually talk to them about your atheism but never try and force it down them. NEVER be critical of Watchtower! In fact, it’s possible to be critical of god or Jehovah, but once you start being critical of the organization then your ex and your children will see you as a danger and your future with them will be under threat.
When they are older they will respect you and can make their own decision. In the meantime, your children will be encouraged to spend time with you and to honor you even if, sadly, the ultimate aim of mother and children will be to ‘win you without a word’ or in other words, to convert you to Watchtower.