The conjunction 'if' and the modal 'must' are correctly used in the sentence, aren't they?
If parents want their child to be healthy, they must encourage him/her to consume the required number of calories.
Rommel If parents want their child to be healthy, they must encourage him/her to consume the required number of calories. Fine. CJ
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RommelIf parents want their child to be healthy, they must encourage him/her to consume the required number of calories.
Fine.
CJ
Rommel him/her
This is possible but wordy. Them would be sufficient.