This sure hits me personally. I also requested that no one give my children pop/soda when they were young. I am shocked that anyone would consider giving the baby coffee! Firstly, I cannot understand any reason why people would want to do this, giving empty calories, gaining a taste for something that serves no purpose, and filling the belly so that nutritional food has no room.
This is going to require everyone to get on board. Ask them directly, what are you gaining by disobeying a rule that I have given about my child? Do I sneak behind your back and do something with your child that you would prefer that I would not? Ask them bluntly, "How does my rule against these items seem so unfair that you would disrespect me act against my wishes?"
I'm sorry to say, you need to make these people uncomfortable with their actions. It is absolutely inexcusable that they would not only act against your rule, but for something that does not benefit and is actually harmful to the baby!
My youngest will be 9 next month and my oldest just turned 11 this month. Both of them have the perfect weight for their height. Neither has received pop on a regular basis, and if I were to put the total amount of pop during their lives, they probably haven't had more than 2 cans total. They love milk, flavored milk, cranberry juice, regular water, and flavored carbonated water.
Compare this to some children that have had a lot of pop/soda over their lives. They may be overweight, have behavioral problems because their bodies build up a need/addiction and rebel when the need is not met, and may experience the high when drinking and a low when their body crashes. This does not mention the affects on their teeth. All that sugar laying on their teeth until they are brushed will work the cavities. Dentists are not cheap, and getting cavities when you can prevent them is silly.
I just do not understand why anyone would choose to disobey you on this. My best to you on this situation. I was fortunate that my family stood behind my beliefs on this (with some grumbling about being a bit of an overcontrolling mum, but after I put the pressure back on them on why I SHOULD allow this, they had no counter answer). I STRONGLY ADVISE that you always err on the side of caution when you are dealing with your child.
I also pulled the BIG CARD out and stated very clearly that if my wishes are not followed, I would not allow unsupervised visits with the children. I would not allow my children to be with someone that was actively working against my rules and/or damaging my children's health, which is what I believe this to be. This might be very difficult for you if you rely on these people to childsit for you, but I'd find a stranger that will abide then someone actively working against me.