I understand your want to keep your "unknowing" preschool children out of the complicated parts of a blended family, but they are also part of the blended family, even if they live with both their biological parents.
It's important to help all children understand that, although it's not the ideal situation (divorce and remarriage) the kids have done nothing wrong, and there's nothing wrong with the teenage step daughter talking to her bio-dad on the phone in front of them. In fact, you can help your teenage step-daughter explain to the little ones the relationship. Talk in a positive manner, about when two parents divorce, and then remarry- now there is a posibility of two moms and two dads, a biological parent and a step parent, all available to love and care for them.
I know it's difficult, but to have a successful blended family, it's better to be honest and forthcoming to all the kids. The little ones will understand more quickly than you may realize, younger kids naturally want to see a mom and dad in their home, and even if they travel between homes- it still makes sense to have a mom and dad in each home.
Shirley Cress Dudley,
MA LPC NCC
Director of The Blended and Step Family Resource Center
Blended family and single family expert