On Thursday, there were so many children messing around that we couldn't supervise them, as we were only two people.

messing 是动词 mess 的现在分词形式，mess 本意是"弄乱、搞糟"，在这里指"胡闹、捣乱"。 around 在这里作为副词使用，表示"四处、到处"，与动词搭配时强调动作的无目的性和随意性。 messing around 是一个常用短语，意思是"无所事事地闲逛、胡闹"，指没有明确目的、随意玩耍或捣乱的行为。