Tommy, fantastic that you only blew the whistle twice. Seems like you had an easy game, particularly with the age group.
Last season, two players going at it at the same time (both fouling each other), if you needed to stop play, the restart would be a dropped ball. However, this season it has changed. You now have to punish the most serious offence, so the restart would have been a direct free kick or penalty kick. Often, play unfolds how it has for you and you can continue. I usually shout "carry on" to see what happens, it gives me a bit of thinking time. However, keep your trailing eye on the players. If one of them is a little naughty, there's your more serious offence. It is a tough one but if you think someone has led quicker, someone has gone in a bit harder etc, there's your more serious offence. Thankfully it doesn't come about all that often.
That's a tricky one for me. If the player has slowed down because he's tiring, it can't be helped, so long as he doesn't commit a punishable offence, he's not contravened law 12. However, if he has deliberately changed his pace to interfere with the opponent (in an attempt to impede or foul the player), you could call it. Have a look at what the arms are doing aswell as that could be an indicator, aswell as the distance to the ball. These ones are difficult to decide when you haven't seen the incident.
Judging by the way the game went for you, it sounds like you handled both situations very well.