Here's what I shared about dicing an onion. Always use a very sharp knife to cut an onion. The more pressure you put on it, the more juices and aroma will fill the air. A sharper knife results in less pressure, thus less juices to make you cry. Try having a lit candle beside your chopping board.
Don't cut all the way through the onion.
Cut thin crosswise strips
Then cut the onion in half and turn sideways and proceed to cut the otherway.
You have a stable cutting side when you cut the onion in half
*The smaller you cut your crosswise strips the smaller the onion will be diced. Children notice it less in recipes when it's diced small.