The people I know who say, “don’t drink water before a meal,” or “don’t drink water during a meal,” tend to have a lot of old world thinking, and old wives tails in their heads.
For children I can see it, if the child seems to not be eating well, but that is very doubtful to me if we are talking about water. Children often get dehydrated easily, so denying a kid water if they are thirsty doesn’t sound like a great idea. Juice, milk, Koolaid, and soda, are a different story because of the sugar and calories. Then the child might actually fill up from calories.
Edit: for me personally, water I drink during, or just before a meal, help me. When I drink water not during a meal I just pee it right out 30 minutes later.