I havent seen one above 2.5
We use a 2.5 one all winter but add a sheet on top and even one thin blanket when its really freezing.
Also, you can just adjust the layers of clothes they are wearing underneath. DD wore a vest, sleepsuit and socks under her sleeping bag in winter. Personally I think if you go as far as 4 tog they will get too hot in the early night while you are still downstairs and the heating is on. 4 is the maximum tog a child over the age of 1 should have for their duvet, so might be a bit hot for a sleeping bag.