A.T.
Well when children play in the water in pools, water goes up sometimes from the nose to the ears and bacteria/viruses in the water may cause a cold and/or an ear infection.
It happened a lot with my daughter when she was little, she got an ear infection each time we went to the pool during the winter so we stopped... She outgrew it later, she's 6 and had only one ear infection last year after going to the pool once again. Nothing this year !
I asked the pediatrician about ear plugs but she said the trouble comes from the nose when the water is inhaled so a nose clip would be more useful than ear plugs.
Some children are more sensitive than others too...you can help if you give them lots of fruits (for the C vitamin), teach them (the oldest ;-)) to blow his nose, wash the little one's nose with a saline solution but other than that there's not much to do...they're also building their immunities...