What is the reason the mesentric nodes enlarged in children?

Mesenteric nodes: These nodes can react to bacterial, viral or abdominal infections just as lymph nodes in the neck do. They also react with malignancies.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells/digested food. Lymph nodes "taste" the lymph everywhere for bad stuff like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.