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.