Congratulation first on your wedding and the big day that is coming up. Can understand how busy you must be getting prepared and being sure everything is just right for you and your guests to enjoy.
Without a doubt you are not a bridezilla after reading what you have stated on the busy events that will be going on that day. So much will need to be done that morning as having all the bridesmaids getting ready as well as any last minute things that need to be addressed. Having this little boy in the room can make some uncomfortable as well as can cause needs to be addressed (bathroom visits for example). The mom it seems is taking control of the situation without taking in the wishes of the bride to be. I am sure the child is well behaved but with such important things going on that morning that having him in the room is not the right thing to have.
My thought would be to see if a babysitter can be used from the hotel and possibly pay for this as while it is an extra cost, the price of having him taken care of properly would put things at ease. Being a single mom, she might not have the money to pay for this so consider it a cost worth the money.
A six year old should also be sitting at a table with his mom as well as no need to have him sit at the head table as he is not a relative. The bride and groom make up the seating chart and your wishes are first and foremost important. Not going to make everyone happy at an event such as this but you are correct that he should sit with mom at a table not reserved for relatives.
Go ahead and hold your ground and speak privately with the mom and explain what a busy day it will be and that her son will be taken care of properly with the babysitter. Offering to pay for it can help in making her understand.
Please let me know of any other questions or concerns you may have on this topic. If you could take a moment to rate my assistance so that I can receive credit for assisting you as well as allow me to help others on our site.
Best to you on your big day.