Do the best you can.
Children are so unpredictable, in their eating, growing, running, lazing, that personally, i think tight control all the time is a futile exercise.
One day they're ful of energy, running around all day, the next, they do nothing and aren't hungry, day after very hungry eat you out of house and home and then just a few more biscuits.
What insulin reigime is your child on? There are different benefits and fallbacks to each, and they all bring their own issues.
Generally, i'd advise you to look at things over a week / few days. If something has been consistently high / low for a few days at a time, then it's probably something that needs to be changed.
Micro-managing diabetes for children does more harm than good, i.e monitoring absolutely everything they eat, never sweets, never chips, tests every hour etc.
If your child was recently diagnose they're stil most probably in the 'honeymoon' period. This means their pancreas is sporadically producing insulin. This really knocks 'tight' control out of the window, as there's no way to know when and how much the pancreas will produce.
Do the best you can, while still letting your child enjoy themselves. It's a very hard line, and it is hard on the family.
Good luck