I'm not a fan of Freehold estates.
O to A for life, then to B for life, but if B gets divorced, then to C for life, and when D's oldest child is 21, then to D's oldest child.
A life estate, B vested remainder subject to condition subsequent, C has shifting executory interest, and D's oldest child (assuming she's already born) gets a springing executory interest. Or some crap like that. Stupid freehold estates. I can't even look at the Rule against Perpetuities right now.