35L. It's a nice focal length for environmental portraits (esp. at night) and for use indoors, where there is typically less space. You can always crop the images a bit in post. If I'm outside where space isn't an issue, I'm usually using longer lenses than the 35 or 50 anyway, especially for children who tend to move a lot.
Both 35 and 50 are sharp enough for portraits. For use stopped down, your 24-70 II is going to be able to match the 35 and will be sharper than the 50, so the instances where you'd use a fast prime would be for shallow DOF or in low light, where corner sharpness is not an issue.