There are many large people who are able to do things well. That should not really be an issue for you, unless you know that THIS man cannot do the job. You seem to be saying that YOUR day care has safety issues and that you suspect that they also hired someone who may not be able to do the job, which may or may not be the case for this particular man. If you know that this man cannot carryout the job and puts children at risk, then you should say something about his ablity, not his weight and all the "what ifs." You seem to already have plenty of reasons to complain, why focus on the big man? Safety is safety...if children are not safe walking through this parking lot, they are not safe, large man or not. There are plenty of slow skinny people, or skinny people who do not pay attention, or skinny people who might trip over kids, and the force of thier skinny body would be dangerous too...
I should say, I am a large person, but I am active, and I function well. I have never lost a kid, nor fallen on one, and most of us know our own limitations, I probably would not tiptoe around a bunch of sleeping kids anyway, and maybe nobody should...which brings me to another issue you should be focusing on...kids and adults of any size should never be crammed in anywhere. It is a safety issue in and of itself, and if this day care is in a public school, there are very specific square footage per child requiredments in building and education codes. Call the fire department and report this today, and when you do, take a look at the size of some of the firefighters...while not fat, I am sure that you will find some of them to be larger than you would expect, given the work that they do. You may want to reconsider assumptions based on size, and move more toward individual performance.
I hope you take steps to see that all these kids are safe. You will be taken more seriously (and might get help from the large man too) if you focus on things that have nothing to do with personal size.
M.