Tables are sent up in even amounts: 2,4,6, or 8 and so forth. A table for 6 will not fit 7 even with a 2 year old.
You can check the Disney dining website for openings for 7 without cancelling your existing reservation for 6. It will show available times for a date and time that you pick. Usually, if your time is not available then it shows you other options if there available. You may get your time to your new party size or you may have to choose an alternate time.
Le Cellier can be a hard reservation to get or change. If you don't have good results you may want to consider booking a different restaurant that can accomodate your new revised size.
If you try to just show up you are probably going to be treated as a walk up and that could take forever. In many instances walk ups are told there's no room at Le Cellier. It somewhat depends if you are going during free dining or not or peak season and so forth. Obviously ressies are harder to change during those times.
You didn't say when you are going. If you are far enough in the future you have a better chance of changing your reservation. You can call and ask.
Another stradegy may be to break up into two parties and see if you can get two reservations. So, a party of three and a party of 4 or a party of 2 and a party of 5 may work. With the 2/5 combination you can keep your table of 6 and then you only neeed an extra table for 2. You will need a second name for the second reservation.
Also, you can keep checking for a reservation cancellation each day. There are cancellations that may help your cause. People cancel their trips and hence restaurant reservations may open up. The best time to look is the 45 day mark when final payment is due for a trip. That's when some people cancel; because, it's do or die time.
Good luck.