K.T.
well it all depends on what your hubby is going in for (my dad had cancer and i was the one to take him to chemo) if its serious id say you would be in the "wrong" not to go. yes your child is sick but if your hubby is as well....can grammy or auntie stay with her? so you can go with him?
ask friends if they can stay with her (not the best option but if she can get sicker it might be the best taht youve got.)
if its nothing serious i say let him go alone-he may not seem like it all the time (mine doesnt) but he is an "adult" and will be at his mommies house so he should be fine.
I hope this has helped some,
K.