iPad plugged to charger turns off due to low battery
An iPad needs twice the power of an iPhone to charge.

A phone charger will slowly be able to charge an iPad, but possibly not be able to keep up whilst it is in use &amp; doing intensive tasks.

It's safe to swap them, an iPhone using an iPad charger will not be damaged, but as you see, the other way round, the iPad is not getting sufficient charge.

Even switched off, the iPad may show 'not charging' at the top right - but it actually will still be charging, just very slowly.