Prices are dynamic, so that's hard to say what the price would be when she knows what her time off would look like.
If the price is not tremendously different, and you get a good bargain (esp if there is a 'kids sail free' promotion), it may be better to add her now, and either remove her or leave her later, depending on which is the better price at that time. If you leave her on, and she is a no-show, you will get the port charges back when you sail.
Also, if the ship is full, you may not be able to add her later, if there are quite a few cabins that have more than double occupancy, and they have hit the limit for that muster station that the cabin is in. People have had this problem before.
If it was my family, we would have all on at booking, work it out later.