Children are, on the whole, much more susceptible to potential damage from their environments. There are all sorts of reasons for this, perhaps the most obvious being that they’re much closer to the ground than adults are. Children spend more time outdoors, getting mucky and generally coming into much closer contact with their surroundings than most adults ever do, they also have a larger surface area when compared to their body masses which means that there’s simply more of them to get exposed to potential allergens or dangers in the first place.
The factors above are why asthma is more common and can be more severe for children. Allergens aren’t the only dangers in our environment however and since the widespread use of pesticides and toxins in our gardens and the world in general, there are all sorts of nasty things which children can come into contact with. Poisons are always less effective on bigger people. The larger you are, the more of your body there is for the poison to dilute in and, as such, the less potent and potentially damaging it will be. Not only are children tiny, they’ve got less developed immune systems so not only will toxin affect them worse than adults, bacteria and similar will do too.
Now, it’s important not to over-react here. Over protecting your children from the world can lead to problems as they’ll have under developed immune systems and be unable to fight off bugs and illnesses as well as they should be. Just make sure you don’t let them play in places which have been sprayed recently and teach them the importance of hand washing after playing outdoors.