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My recommendation is to change to a healthy diet so there won’t be any need for tests. Increase the amount of plant-based foods, limit processed and unhealthy food. With a healthy diet you won’t lack any vitamins or minerals and won’t need tests.
This barely answers your question but is important nonetheless; The gold standard for feeding kids:
Parents get to decide what food is being offered (nutritious, balanced and without villanizing any foods) where (hopefully at the table with the family eating) and when (a regular routine schedule)
The child gets to decide if they eat the foods offered and how much. No pressure no bribes even limited praise. Neutrality and leading by example is your best bet
This helps them self regulate their appetite and weight which is the greatest gift you can give them into their adult lives
If you keep all the unhealthy foods from them, or label foods as good or bad, or talk about diet and health too much - they will rebel and want the bad foods even more. These behaviours are what will turn a chubby and otherwise healthy kid into a person with a lifelong problem with food and weight
One surprising thing that helps with appetite is to give them a little candy, maybe half an hour to an hour before meals. This will spike insulin, increasing appetite.
Focus on protein first for their nutritional needs, salad is the number one priority