Bchasdei Hashem our child had a transplant. B”H made a kiddush. Is there a requirement to make a
Seudas Hodah for a child or minhag?
Also when is considered healed after a transplant? As they normally need to take anti rejection medication.
No, there is no requirement. This is a praiseworthy minhag, but the point is to publically thank Hashem for the great kindness He has done for you. So a kiddush would suffice to fulfill this minhag.
At which point the doctors consider him out of danger, would be the proper time for such a seudah.
See Mishna Berura 218:32, who mentions a number of other suggestions, such as giving tzedakka, doing something to benefit the public or Torah learning. And of course every year on that day to thank Hashem for His kindness and relate it to others as well.