OK I'll try to give you an example. However, just a bit of advice: this type of question is VERY common and among the easier of CR questions. If you do not understand it properly and pretty fast, you might get into trouble later on. It's doable! You can do it!
You have the following case: the school had 500 children ten years ago and has 500 children now as well. At present, 10 kids come to the nurse with allergies. In 2000, only 5 kids came to the nurse.
The proportion is obviously bigger now than 10 years ago, since 10/500 > 5/500.
So let's take C and negate it. We have:
Children who have allergic reactions to the chemicals are more likely to be sent to a school nurse now than they were ten years ago
This tells us that the kids' supervisors are more cautious today than they were ten years ago. This could mean that ten years ago 11 kids were allergic to the substances, but fewer were sent to the nurse because teachers were not paying attention. If this is the case, then the argument falls apart because there is an ALTERNATIVE EXPLANATION to the increase in proportion: teachers' care.
About E: I never said E is about other children. The argument concerns strictly schoolchildren and the proportion of allergic kids among schoolchildren. Proportions involving something other than schoolchildren are irrelevant to the argument.
E is playing on your attention. The argument strictly says "proportion of schoolchildren sent to the nurse". E suggests that the population might have changed, so the total number of kids sent to the nurse is different. However, the TOTAL NUMBER is not important, it's just the proportion.