The answer will very much depend on the quantity, type of juice (pure orange or sugar/synthetic), and where the spill is. If modest quantity much of which was soaked up by bedclothes and it is near an edge, then tip the matress up on its edge and spray with water (pump action bottle is ideal). Once well soaked locally, use a towel to dab/press and collect the juice. Repeat several times. If the mattress is not a spring type (e.g. foam) then take it to a garden hose point and drench while it is in a position whereby the area drains down. Obviously, the mattress will not be usable until it is dry again. Finally, teach daughter not to carry food or drink into bed.