It took me this long to understand something which should be so clear by now.
Children has no sense of time. They instinctively knew what we have to learn again and again - to live in the present, the "now", and not some past or future.
When they leave and seemed to forget the time spent together with us, it is not that they're forgetful or 'heartless', but that other things have occupy their time.
This 'other things' are always, always important to them precisely because they're fully experiencing them at that moment. These things or experiences, they're what we called "life".