Dan A, you are about to embark on one of the most rewarding pastimes known to man - refereeing rugby in England at U.13 and U.14. These games can demonstrate extraordinary skill, but they are also (mostly!) within the physical parameters of a not-too-unfit dad. My advice would be:
a) research the
age grade variations on the U.13 handoff. As far as I know, there is no law variation regarding the hand-off at U.13 which means that it is as legal at U.13 as it is at premiership or international level. There is, however, a widespread misunderstanding among those familiar with U.12 variations that the handoff is banned at all junior levels - or that it is permitted to the chest but not to the head or face. it makes sense to consult with the opposition coach and vary your approach accordingly. This means that your team will have to react to the ref on the day - prepare them for this.
b) while some players are highly capable
(check this out!), others are relatively new to the game. All will find that the space on a full-sized pitch means that the game is very different - and that extra space will tire them out even in the short matches they play. Be prepared to coach as a ref - even in formal competition games. Try to avoid the penalty if at all possible, but give it when fairness requires.
c) Persuade your team management to invest in numbered shirts - the increasing importance of these games to both players and parents makes life for the ref very different if all you've got to go on is that the medium build kid with the average haircut trod on the opposition rucker.
d) Consider joining the local refereeing society. Goodwill and a few gym sessions will see you through this season. Next year one of the boys will outpace you. The year after that, you're screwed unless you take serious steps to develop your speed and decision making. Join the Society, take it seriously and you're good at least until you hit 50 (and perhaps years thereafter).
e) Don't be a jobsworth, and resolve to have at least as much fun as the players.
f) That said, be prepared to ping the "tap ball on foot" penalty. If they don't learn this skill now, scrum halves will still struggle with it years from now. Do them a favour early, and be harsh on the quick tap penalty after an initial explanation and opportunity to retake.
I envy you being at this stage. Enjoy!