As a 17 stone 43 year old with 2 dodgy knees and a temperamental ankle, I for one appreciate any opportunity for additional fluid intake!
During an u14s game I did this season just gone, there was a lengthy stoppage due to an injury and several players from both sides took the opportunity to grab a drink on the sidelines. One coach took exception and protested at the "unofficial drinks break". I however had no problem with it at all. Yes, technically players did leave the pitch without express permission, was I bothered? Not in the slightest! Anyway, all players had returned to their positions on the pitch before the player had finished receiving treatment, so it made absolutely no difference to the length of stoppage in play whatsoever.
There have been times where I have arrived at games and suggested/asked managers regarding drinks breaks, other times they have asked/requested first. Would have to be a fairly petty minded ref/manager/coach who objects to a drinks break, especially at youth level. The wife and I usually have to nag our son to drink, I for one am not going to stop a kid getting a drink even if it is mid-match!