I haven't been there since they implemented the switching desserts to a side deal on the DDP, but I have read that they are allowing people to do this. It may just come down to who is working when you are there. That being said I don't think you should have any issue with doing what you are planning, and I am sure if you told them that you are ordering the shake as your beverage, but wanting to give it to your kiddo as his dessert they would wait to bring it to him whenever you are ready for him to have it. What we have done in the past is ordered a shake as a bev on the adult plan, and ordered another shake as the dessert, and just our server bring out all shakes at the same time, as dessert, and we just drank water with our meal.