I quite like the rule one poster said about 'no breakfast, no practice' in theory, however as a child I would have hated it!
I am not a big morning eater now, and was even less of one when I was younger. Eating before I was ready just made me feel sick, then I probably wouldn't have wanted to go to practice anyway!
Now, as an adult, I am obviously more in control of what I eat and when. I know that I generally can't eat until 2 hours after I've got up so making sure I'm up early enough for my stomach to wake up too is useful unless that would be a ridiculous time! I like to have a variety of breakfast foods in the house because I can also only eat what I fancy (I'm really not a fussy eater any other time of the day!) but I generally have the following available: cereal, bread, crumpets, eggs, English muffins, fruit, yoghurt.
On days where it would be silly to get up 2 hours early, I try to take something with me like a banana.
I think a lot of coaches tend to ban gatorade/powerade type drinks because they are so sugary and we want to promote healthy lifestyles. They also stain if spilled! We only allow water at our gym - we have a water machine so gymnasts don't even need to bring their own or leave their bottles behind!
As far as snacks, our developmental gymnasts aged between 5-7 all have a 5 minute snack break for every 2 hours they are at the gym (it must be something healthy)
For gymnasts training longer hours we discourage snack breaks however wouldn't ever ban them altogether and if a gymnast needed a snack for any reason they would be allowed to have one. Our longest sessions are 4 hours long - some gymnasts do train 6 hours on a Sunday but with a proper lunch break.
I think most clubs/coaches are pretty reasonable - some just need educating - not all coaches have their own children or know/understand about little ones needing to eat regularly! Speak to your HC and explain the situation, you never know until you ask!