What would be the most common longterm effects of malnourishment during childhood?

Many: This can be extremely variable. Depending upon the severity (eg third world starvation will be much worse than us poor diet choices)-- there may be anything from no symptoms, to dental disease, to mental retardation and growth failure. Luckily children are resistant and if the malnourishment was short-lived, sequelae can be minor or absent. Ask your child's doctor regarding their specific case.