Oh my word, please stop yelling at them. That is not nearly important enough to yell about. Yelling will not work. They need to learn to like mealtime, it needs to be a pleasant experience for them or they will have terrible eating habits for life as a result. Stop yelling first off.
Children eat what their bodies need. Offer a well balanced meal at every meal and they will eat what they need. There is no reason to try to force them. I think I would offer more variety long term. For short term theough, it is a phase, it will pass. In the mean time, find a few staple foods that they love and offer them frequently. There is nothing wrong with milk. That toddlers growing body needs all of the calcium and vitamins in the milk. And the pears are healthy too. You didn't mess up by introducing crackers and cheerios. I think you could find the healthies crackers possible ant try to get them to eat those. The cheerios are good for them also. They have vitamins. If I were you I would go purchase some vitamins for them. That would ease the stress of trying to make sure they eat all of the vitamins they need, because they would be getting more in the way of vitamins. Another suggestion is Pediasure! It tastes great, and has all of the things they need, maybe you should substitute ont of their milks a day with pediasure. You can ask your doctor about that.
Whatever you do don't yell. This phase will pass. It is not worth getting worked up about. They aren't going to die from drinking too much milk or eating too many pears. Their little bodies will tell them when they need more. A good example is the cravings we get even as adults. I know I personally will crave meat sometimes and I know it is because my body needs the protein. All you can do is offer good foods at every meal they will eat what they need, don't stress. I think the vitamins would help you relax some. Also, it may help if you sit down to eat at the same time they do and eat the same foods that you are feeding them. It may not be such a struggle if they see you doing what you want them to do, they may just follow your lead!