How strict are you about dietary guidelines when you visit?
VERY strict. Especially with my monster-in-law, who thinks food is love and therefore crap junk food must be extra love (heavens knows why). Actually I make it quite clear I don't like DD having processed foods, I'd prefer fresh and healthy.

But it's easier for me, we just don't go over there for meals. We schedule morning/afternoon tea instead and I take along a plate of fruit. When DD is offered the various little cakes she has made I limit her to one and then ask if the plate can be put up out of her reach (any excuse works here, it's too close to her lunch time and I don't want her to spoil her appetite, last time she ate too much cake she threw up, whatever works... not necessarily true).

Another tip might be to hunt down some of the packaging when she's not around, find a particular additive that seems to be in everything you're concerned with and blame it on a negative side effect in your kids. Then just say they can't have that additive and can you check it isn't in the food being offered. Better still, have DH say that and then you're not the bad guy.