Protein deficiency means that the kid is not getting enough protein through their diet. Protein deficiency is very uncommon. But it may show up in some cases like, if a kid has some medical condition, they follow a vegan or vegetarian diet etc. In most of the cases this deficiency can be met through a balanced diet. However, in medical conditions,  their doctor may prescribe supplements. Protein deficiency may show up in a kid as slow growth, anemia, water retention or edema, weak muscles etc.