What to do about in-laws that won't follow my rules?

[Julie]

Long story short, my in-laws are a nightmare. We live a block away and I have a two year old daughter, so we are between the two houses constantly. The problem is them not following our rules in reguard to what we want our daughter eating. I never had weight issues as a child, so I am mostly just concerned about her developing healthy eating habits for life. My husband, on the other hand, grew up in a house where everyone over-eats and basically eats junk all the time. He was very obese as a child and has had to work extremley hard to get fit and stay fit, so he is also concerned about our daughter learning the habits that he learned at a young age and having to go through everything he did as the "fat kid" at school, then having to feel deprived of something when she has to change her eating habits in the future. We have been very direct in conveying our rules and they blatently disregard anything they think is "silly". Don't get me wrong, we're not food Natzi's or anything..
We just have basic rules like only one snack in between meals, we eat meals at the table, one treat after dinner, milk or water, no juice, things like that. But every time I turn around someone is sneaking her snacks behind our back. I have been good natured up until this point but I just walked in on his grandmother feeding her a jumbo sized piece of key lime pie ten minutes before her bed-time. I about blew my lid. I don't know what to do short of just not taking her there anymore. I just picked her up and left because I didn't want to say something that I would regret later, but I'm not sure how I could be more clear. I'm ready to tell them that I am going to start paying a babysitter to watch my daughter when I am not home (usually she stays with them) just to make sure things are done the way I want. It's been causing her to have terrible tantrums at home now because she doesn't get the things she gets there. I'm at wits end...any ideas or help is appreciated, thanks!
Wow...thanks to everyone who answered. I'm working my way through reading all the answers now, but I just wanted to comment on a few...

First of all, this is not an occasional treat I am talking about. This has been going on since my daughter was three weeks old and they tried to sneak rice cereal in her bottle. At three months old they started with the juice and the solid food. I asked them repeatedly not to per doc's orders (really) but continued to find baby juice in the garbage and chunks in her formula. It only got worse when she began to eat real food. Now she expects a cookie every time she walks through the kitchen.

I don't give my daughter juice because her pediatrician specifically recommends against it. It is all sugar and empty calories. She eats plenty of fruit for vitamins and also takes a toddler multivitamin everyday.

Finaly, I do not "take advantage" of their free babysitting. I have a job that allows me to bring my daughter with me, so I rarely ask for...
them to watch her, it is very much the opposite. Her grandmother calls me nearly every morning to ask if I can bring her by. I can do just about anything I need to with my daughter so dropping her off is not a necessity, I do it as a favor to them because I know they enjoy her company. Even when I sit in the house with them grandma will sneak my daughter things, literally while my back is turned so I feel like I have to follow her from room to room. Because we live so close, she can look out the window and see if my car is home so when I don't answer the phone she just walks over. Grrr....

Once again, I am not a food nazi. I am all about letting kids be kids, but I want to also instill healthy habits in her now so that when she does have to make choices for herself hopefully they will be healthy ones. Family medical history includes diabetes, heart disease, obesity, high blood pressure, high cholesterol, etc. and that's just one generation.