A house with kids in transition, sleeping from the crib to an actual bed, will do well with a bed convertible like this piece from Ethan Allen.
The bottom bunk to this bed can be adjusted to become a full-sized sleeping space for the child, with the bed serving him well up to his teenage years. Additionally, the bottom bunk has extended legs, support rails and brackets, as well as additional hardware to complete the transformation of the bed.
The conversion kits are sold separately, but this will be very useful for when the children begins to grow up. It would be more practical to have this, especially when the kids express the desire to be sleeping on their own, and grow out of that awkward stage where co-sleeping or crib sleeping no longer cuts it anymore.
You can get this from Ethan Allen.