to want to make a change? (visiting / illness)

Tricky balance. It all sounds too intense to me.
You need to be creative in coming up with acceptable alternatives that are win win. Tactful and devious sums it up! It sounds like you need to lighten up a bit too. Germs happen and immunity is built by exposure unless there are medical complications or people are very frail/elderly.
4 days twice a month is too much and the regularity is opressing.
Try inviting old friends to stay so that from time to time there isnt room, or take in some paying foreighn students if you need cash! You need to beak the pattern of increasing dependency and share the load with other family or friends if you have them.
Perhaps get out more yourself so that your toddler days are filled elsewhere, then you wouldnt be home and available.
From the granny side, i dote on mine too and as they get a bit older they come to me to give the parents a much needed break! I do want to see more of them but have to respect their private family time when SiL works so hard all week.
One lot live two hours away and if I didnt drive down there I would not see the little ones growing up. It's a long day but I day trip occasionally, meet for lunch or after nursery, then drive home after tea! Its hard for them with both parents working!
Hmmm good luck and be kind to an obviously lonely granny. Find her some other interests too

Mishap I have just visited this thread. I love your philosophical view of living with small children.

"Life becomes one long snot trail and saga of sleepless nights and buckets of sick - that's just how it goes I am afraid."

Not for me I hope.