During Jesus’s time, children were “considered to have low social status and were often overlooked.” They represented “innocence, humility, and the least in society.” (Bible Hub)

The disciples were arguing amongst themselves about who was the greatest. So in order to teach them a lesson, Jesus took a child in His arms while talking to them. He told them:

““Whoever welcomes one of these little children in my name welcomes me; and whoever welcomes me does not welcome me but the one who sent me.””
‭‭Mark‬ ‭9‬:‭37‬ ‭NIV‬‬

Jesus taught them that when they humbly receive with gladness the lowly and the needy, it is as if they were humbly receiving Jesus Himself. If they received, or welcomed Jesus they welcomed their Heavenly Father as well.

Our lives are not to be spent working to achieve a position. God values the one who, in humility, serves not expecting anything in return. It’s the little acts of kindness, giving “a cup of water” (verse 41) that God rewards.