What's the safest way to treat child with low thyroid?

Thyroid hormone. The only way to treat a child with hypothyroidism is to replace the thyroid hormone which is lacking. Levothyroxine is very safe and is used in newborn babies who are born without functioning thyroid glands, too. Growth is dependent on getting the right amount of thyroid hormone. Brain development is dependent on getting the right amount of thyroid hormone particularly in children 2 and under.
Levothyroxine . Levothyroxine is the drug of choice. Labs should be followed closely.
Thyroid replacement. The first thing to do is to confirm that the child has low thyroid. This should be done by a competent, board certified md/do. If the diagnosis is made and confirmed, then the next step is to begin treatment with a fda approved thyroid hormone pill. Keep close follow up with the prescribing physician. Good luck.