The first thing that should reassure you about this event is that few people will be listening very closely to you. Kids will be enjoying the sports activities, and parents will be focused on their kids.
The first thing that you should be aware of is that although your putative audience is the kids themselves, the most important audience is actually the parents and school staff. An eight-year-old is unlikely to complain to the school board, while an aggrieved parent or disapproving staff member can make trouble for you.
My first suggestion is tone. You should be cheerful and upbeat and praise everyone. You should be especially concerned to make the losing children feel happy and say something about how hard they tried or some specific good thing they did. The winners will feel good because they won, but the losers are more likely to cry, sulk, or be insulted by their peers.
Next, you might use references to popular culture that the kids will understand, such as quotations or examples from the Harry Potter series, Star Wars, or other popular works. You don't necessarily need exact quotes.
You should use simple language but should avoid irony, as that might be seen as mocking the children. Not offending anyone is a higher priority than being clever.
On a practical level, make sure to speak slowly (more slowly than natural speech), avoid profanity or insults, and write out the major facts in advance (the length of races, names of participants, etc.). Make sure to say everyone's name at least once. Thus, you might say "leading the 100 yard run is X, followed by Y, with Z in the outside lane catching up to the leaders, followed by A, B, C, D, and E. Worry more about being fair than about being clever.
Perhaps you could adopt a humorous tone since the children are so young that there will be no spectacular competition. For instance, when the three-year-olds participate in an acitivity, you could "explain" that they have trained hard for this event by drinking more chocolate milk and doing lots of bench presses with pints of ice cream (or some food) and leg kicks right before nap time.
Another thing that you could do is watch ESPN for a few days and learn some of the sports jargon. Then, you can apply this jargon to the children's events and their actions. Doing so will give the children's little events a hyperbolic tone that should amuse some of the parents. Fabricating some narrative about the little athlete who is up to bat, etc. is cute also.