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 your tutees take between 1 and 5 attempts to pass a level, on average.
You may have a tutee, 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 tutee two or three times longer to answer each question:
Go to "Manage Users", then select the tutee'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 tutee's difficulty settings. It will save automatically.
More Difficult
Alternatively, if your tutee 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: