Here it says that your children will ask you Machar. Below (verse 21), it says that they will ask their fathers!
Radak (21): Below, children who will be born later ask.
Alshich (21): Below, your grandchildren will ask.
Malbim: Here it discusses the current generation; below discusses future generations, therefore it does not say 'Lachem'. The rocks are an Os (sign) for this generation, which saw the miracle; they should know that it was not due to their power and merit, rather, due to the Aron's power and the Kedushah of the Luchos inside it (the rocks were from near the feet of Nos'ei ha'Aron). You children will ask why you need the rocks; you need no Zikaron (remembrance), for you saw it! The rocks are a Zikaron for later generations, which did not see it.