Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek urgent or emergency care for difficulty breathing, fainting, extreme weakness, blue or gray coloring, difficulty waking, or a possible iron overdose.
Contact the clinician sooner if the child becomes more pale or tired, eats poorly, develops pica, has persistent digestive symptoms, or cannot tolerate the prescribed iron.
At routine follow-up, families should discuss missed doses, side effects, feeding challenges, milk intake, and any difficulty obtaining or giving the supplement.
If the blood count does not improve as expected, the clinician may check whether the child is receiving the intended amount and look for continued blood loss, poor absorption, inflammation, lead exposure, or another type of anemia.
The article also says
The clinician will usually repeat blood testing after treatment begins to confirm that hemoglobin is improving.
The exact timing depends on the child's starting hemoglobin, symptoms, age, and treatment plan.
Later testing may check whether iron stores have been restored.
Keep scheduled well-child visits so growth, diet, development, and anemia risk can be reviewed.
Every sign above is quoted from the sofidex article on Anemia caused by low iron - infants and toddlers. If you are not sure, err upward — it is always reasonable to be checked.