Are they pushing early because they do not know better?
In which case blow and break up the scrum, have a word and reset the scrum.
Of course if they keep doing it, you have to escalate. This is often summarised as Ask, Tell, Penalise.
Or do they know what they are doing and looking to get an advantage?
In which case blow up and award the free kick.
If they keep doing it then you can upgrade to a penalty for deliberate offending.
If that does not stop them, you might consider going to cards - but probably not in junior rugby.
But usually you are looking for reasons not to blow your whistle.
Provided the scrum becomes stationary roughly on the mark, you may want to simply get on with the scrum.
You may want to mention it before the next scrum