Wall off the hospital he's in. Drop a caretaker into the room from above, along with regular drops of food and drink. Before doing this, however, make a 20x20 room next to the hospital, connected to it by a 1-tile passage. Fill the room with his favorite things - the more ostentatious and decadent the better. If he likes sphalerite and adamantine, fill it with masterwork items made of adamantine. Choose the items accordingly to accompany him into the afterlife - he should have some barrels of sunshine and dwarven rum, and the finest masterwork meals of cow cheese and dwarven syrup your fortress can produce, as well as two masterwork axes (one for each hand), armor, etc., and all sorts of other useful equipment or finery. Also place in it a masterwork coffin made from his favorite metal, and designate it to be his tomb. Make sure that there is no possible reason to enter the room except to bury the child. In the doorway leading to this tomb, place a pressure plate, so that it only activates upon the child's death and subsequent burial. Connect it to pumps that will flood the fortress with magma (except for that room) and collapse a large portion of the fortress, sealing off the tomb. The pressure plate should also activate a repeater that, in turn, activates steel upright spikes that fill all unoccupied space in the tomb. This will ensure that his caretaker will die with him and accompany him to the heavens.