I was an exchange student (summer only) in Japan through Youth for Understanding...um, many years back. It was a wonderful experience for me.
It really did help to have another kid in the family my same age (17 at the time). There was also a brother who was 19. We were both old enough that he and I could go out and do things without the parents always having to accompany us or drive us. So think through how you would handle things if your much younger kids needed to be home but you wanted (or needed) to take the student somewhere -- you and your husband would need to be willing to tag-team things, and you would not necessarily be doing all-family outings with your student as much as you think.
Also, your teen student (if it's during a school year) will need to get to nighttime and weekend school events; other students' homes for study or socializing; the library to do research; the mall to get necessities; the movie theatre to meet friends; and so on. With one toddler and one young grade schooler, are you and your husband going to be up for one of you doing these runs? (Assuming here that your student doesn't drive or is leery of driving -- or his program doesn't permit him to drive while here on exchange.) Are you also up for having teenagers over to your house, and telling your son that he really needs to leave the older kids alone now so they can do their own thing? Or is he going to be disappointed when you have to tell him he needs to leave the kids alone-- or when the student says so, even kindly?
These are things to think through before hosting a student so much older than your own kids. You will need a very good fit, maybe a student with siblings around the age of your kids. When you tell the hosting organization about your younger children, you may find that they recommend you wait. I don't think it's a hard and fast rule or anything like that, but I would guess that teen student host organizations might give preference to families with kids nearer the ages of the foreign students.
I'm not trying to throw cold water on your commendable enthusiasm -- just pointing out that having a student live with you during the entire school year may require more logistics than you expect, especially if the student can't drive himself to school, friends' houses etc. I would do a summer-only exchange that is for a much more limited time and does not involve issues of "We need to leave Student alone now, he can't play, he has to study" etc.See how that goes first.
Exchanges are terrific and I hope you do one but think hard about having someone in your home for a very extended period like a full school year. If you don't mesh or you find it isn't working for your own kids, you will find it difficult to get out of the arrangement and it will be hard to explain to your kids.