Cutting or abrading MDF is risky due to the fumes. In this case thickness would be dictated by the weight to be applied, and I reckon 6mm wouldn't hold up a toddler for long. 20mm would be a minimum for an armchair, but a double or triple sofa width would see it sag and you'd all end up in the middle. I rewebbed a sofa once, and it performed perfectly to the day it was binned many years later. All you need are scissors and a good heavy duty stapler.
https://www.ebay.co.uk/itm/7-Sizes-J...YVDVbMzxTmmkpg