Immunity in children between 2-3 years can be boosted by taking easy precautions and making lifestyle changes, dietary changes, and all-natural nutritional supplements changes. Vitamin C, zinc, elderberry, AHCC, fish oil & flax oil are few things, which enhance immunity.
The other option is to let the children free. Make them in play in dirt and stuff. This too helps in building up the immune system although it may sound a little weird.