Yes, I understand. But the situation you have now puts you in a position where you are asking her every time for the opportunity to see your kids. It automatically puts you at a position of weakness and puts pressure onto her. "Does he want the kids...or is it a ploy to see me?" If you have a regular, fixed schedule it balances that power dynamic, I think.
Initially, W had heavily restricted my time with kids (but that also gave me the opportunity to make the house look much better and try to get my head around understanding what has happened etc.). I knew W would struggle before too long even with the full-time help or her parents. She has now started asking me about seeing the kids more (to suit herself).