That sounds like an okay amount. You will run into more troubles if you suddenly change the amount.
It's hard to give an exact amount because factors such as grain type and it's protein content, what type of hay they are consuming and it's protein content, and things like age and sex and intended purpose of the goat can change many things. A meat goat needs more grain to reach market weight than a pet wether goat does.
I have raised kids on zero grain, but most get a handful or two a day, slowly increased up to 2 cups a day for a few months before I stop feeding it. It can depend on whether the goat is thin or in good condition too.
It is always best to keep grain or rations as low as possible as goats are ruminants meant to eat complex browse. Grain can upset the system and can kill them.
If what you are feeding seems to be enough for them to keep growing without getting too fat than it is a good amount. Adjust as needed based on how they are doing on it and what else you are feeding.