Weight gain occurs during a critical period of the child's development known as the adiposity rebound period . So this could have serious lasting effects.

These results were ascertained by comparing the behavior of 2,055 families and the outcomes for their children. While the increase may not seem like much, it occurs during a critical period of the child’s development known as the "adiposity rebound period". The weight gain could therefore have serious long-lasting effects, the study said.
Pagani has several explanations as to why there may be a cause and effect relationship in the association she has identified.
"Early childhood exposure to second hand smoke could be influencing endocrine imbalances and altering neurodevelopmental functioning at this critical period in hypothalamic development, thus damaging vital systems which undergo important postnatal growth and development until middle childhood."
The results appeared in the journal Nicotine and Tobacco Research.
Source-IANS