One or two Cheerio's a week wouldn't hurt him if he were on a plain dry seed only diet without anything else because that type of diet is extremely low in Iron, but of course that is one of the worst diets imaginable and I'm certain you don't follow it. The dangers you should be aware of with too much Iron is it can cause serious health issues at best or at the very worst it can cause toxicity.
If you feed him a varied diet consisting of Pellets or Nutri-Berries with sprouted seeds & fresh food, or sprouted seeds with a Cuttlebone & fresh food, or dry seed and a Cuttlebone, fresh food & a Vitamin Supplement you really should be very, very careful about giving him too much Iron.
One or two in an entire month (4 week period) could be acceptable if you did not offer him any pellets or Nutri-Berries nor any extra Vitamins whatsoever as they would all have Iron in them and it could be way too much, enough to cause toxicity.
Of course this would mean that Cheerio's cannot be used for a Treat or a Trick Training incentive.
A better treat for trick training would be Safflower seeds since they are large enough for you to hold in your fingertips but small enough for him to eat fairly quickly. A trick training treat should be something a bird can eat quickly or else it really is not very effective. Plain Cheerio's seem awfully large for a Parrotlet to use for Trick Training. Safflower would be much, much better just for that reason alone disregarding all the other reasons.
Purchasing a bag of straight Safflower from a company that sells seed individually is the best way to acquire the seed. It's much, much easier than picking through a seed mix to pull out the Safflower for Treats. After you start using it for Treats it is better to purchase an everyday seed mix that does not contain Safflower because if he can just eat it in his cage then there is not as much incentive for him to eat them from you.
Because Parrotlets need more fat in their diet than a lot of small Parrots, you can also fill up a treat dish with two tablespoons of Safflower once a week to once every two weeks. These seeds, like the Sunflower have the highest fat content but unlike the Sunflower a Safflower is not as sweet and therefor does not seem to be as addicting making it of course a much better seed to feed. While fat is good for beak and feather health it should also be monitored to ensure he won't get too much fat in his diet. He's a Parrotlet so you won't need to be too careful with the fat content that he ingests. You are lucky there; if he were a Budgie you would need to be much, much more limiting of any high-fat seeds.
Dilute-Blue Pacific Parrotlet: Gemenon (Eon)
Dilute-Turquoise Pacific Parrotlet: Prometheus
Cobalt Masked Lovebird: Sapphire
Green Quaker Parrot: Sprocket