Great care has been taken in the design of Story mode to make the mathematical difficulty increase at just the right rate for almost all children.
We should all anticipate a certain amount of "struggle" to pass some levels - that's the nature of learning and improving, of course - so it's appropriate that children take between 1 and 5 attempts to pass a level, on average.
There will be individuals, however, for whom the time constraints required to pass a Story level are demonstrably too tough for them. In these occasional cases, having considered other options such as gentle encouragement, offline help from a grown-up, etc., the most sensible way forward may be to relax the time constraints somewhat for them.
Two and Three Times Easier
To give a child two or three times longer to answer each question:
Go to "Manage Users", then select the child's name.
Click the last tab, "Settings". You'll see the Story mode difficulty is set to "Standard" (highlighted green) by default.
Click "2× easier" or "3× easier", to change the child's difficulty settings. It will save automatically.
More Difficult
Alternatively, if your child is finding NumBots too easy, find out how you can allow them to skip the early levels, in order to reach a more relevant topic, here: