It is natural for everyone to recall what happened just a few hours or days ago. Likewise, it would be too difficult for children to retain what they have learnt earlier.
Yes, all teaching inputs cannot be retrieved at the same instant. However, if the children discover something interesting, it will not be difficult to retrieve. So, before you introduce some new notions, pique their interest. It's also not that difficult to accomplish.
Begin by teaching them in an unconventional way, using a few unstructured approaches to help them grasp inputs from us that they will remember for a long time.