This is perfectly normal behavior for the iPads. If the battery has enough of a charge in it at the time, you can turn the iPad off while it charges. But it does no harm whatsoever for the iPad to turn back on again when charging and it does not add any sigificant charging time to the battery. No worries - all is fine!
The iPads should turn off by themselves after whatever interval you selected in Settings> General> Auto Lock.
This puts the iPad to sleep - it does not power the device off - and that is the OP's concern - that the device starts up (powers up) again when charging the battery - and is not powered off when charging.