Too Hard or Too Light
Any activity that involves modulation of movement — a careful calibration of the amount of force or speed needed to accomplish a task — will be difficult for a child with sensory integration problems around the proprioceptive sense. Whether your child is not getting enough information to know effortlessly where her body parts are and how to move them, or whether she is getting so much information that she can't find the pieces she needs, there are going to be problems with pencils and erasers and spoons and forks and glasses. Your child will have to learn how to manage these things, but the more you can understand what a struggle that is, the better you will be able to help her.
You probably process proprioceptive information smoothly enough that the proper pressure for writing clearly or lifting a glass just comes naturally to you. But think about how you feel when a pen's ink doesn't flow smoothly.
You might write with it so hard that you leave grooves in the paper, with no luck. You may rap the pen hard against the paper, trying to get the ink flowing. You may try holding the pen at a variety of different angles in the hope that something will get the ink going. This may be similar to your child's experience with writing — the feeling that whatever you do, nothing can make the writing come out right.
Similarly, when you watch your child struggle with a fork, think about an experience you might have had with chopsticks. It takes a careful modulation of just the right pressure, and a quick sure movement from plate to lips, to use those utensils properly. Your child may have the same problem with a fork. Drive the tines into a piece of meat too lightly, and it won't stay; drive them too hard, and the meat may scoot across the plate or right off the edge. It's no wonder that fingers seem the more reasonable alternative.
The best thing you can do for your child is to understand that his problem is legitimate and not just a matter of laziness or sloppiness. The second is to see just where and how he's having problems and give him tools that can help. A child who writes too softly may do better with a gel pen in which the ink flows without much pressure. A child who writes too hard may break fewer pencil points if you give him a pencil that's only partly sharpened. A short pencil may give a better, surer hold than a longer one. A soft eraser will be less likely to tear paper when used strenuously than a harder one.
Plastic forks and spoons may be easier for your child to manipulate, or you may want to play with different weights and grips to find a good fit. Food that everyone eats with their fingers — like hot dogs or chicken nuggets, raw carrots or corn on the cob — may be a good thing to work into meals as much as possible. Occupational therapy with a sensory integration approach will eventually enable your child to need these accommodations less, but while he needs them more, try to give them to him.
Remember, too, that the more stress your child is under — the more she is blamed for the things her sensory integration profile will not allow her to do smoothly and accurately — the less able she is going to be to exert any sort of control. Forcing her to rewrite and rewrite, or to use a fork or go hungry, will not get you the sort of increase in skill level you may be hoping for. Don't do that to your child, and don't let relatives or teachers do it, either. Sometimes the biggest hurdle all children with sensory integration disorder have to deal with is not the difficulty with how their brains process information, but the condemnation of those who do not understand how their brains work.