The blood labs are not to monitor toxicity, but to verify absorption. That is far more likely to be a problem than toxicity. You avoid toxicity primarily by controlling the dose.

Richard Carmona mentioned that it took 50,000 IU a day for two months to generate symptoms of toxicity. Undoubtedly less for children, and they resist COVID much better anyway, so they probably don't need therapeutic doses. I've seen up to 15,000 IU recommended for specific adults during pandemic outbreaks, but 2,000 to 6,000 a day is usually sufficient.