Any Advice on how to DM for a 7 year old?

OK guys I need help on this one. The scene and questions below. I just started DMing a 5e game for my buddy and his 7 year old son. In a week we will have our 3rd session (and the son joined us at the 2nd session). I need some advice on how to DMing for younger players. I've played along side this kid before and have noticed it can be hard to keep his attention and also generate original creative ideas outside of combat (understandably) I noticed I'm in the unique situation of being able to DM for a father and son. How can I use it to help them bond and both grow? I know his son likes rolling dice but please help me out so my game doesn't turn into a murderhoboing session.

The Characters, (my buddy) The Honourable Human Paladin who discovered his love for the sea in the cities Navy and now pledges his honor to the sea god Elara

(his son) The Dragonborn Rogue Assassin stolen and hatched from birth by trolls and now has accepted his first bounty from the town they are in to kill a local outlaw leader

(potentially turning this guy over to the 7 year old..)(?) [NPC] An unknown Half Orc Fighter with a unjust kill order on his head that the Paladin saved from the noose when the zombies showed up

(Pot. same with this char. to my buddy...) [Undiscovered NPC] Human Wizard / Drow Druid (not Introduced yet)

So to start quick, Big suprise the town has zombies rise from the storage catacombs running under town. The Paladin, the Assassin and the Half Orc kill all 5 of them and go downstairs. There are about 4 zombies in the in the catacombs and they kill them fast. (Ok I summarised alot so there was alot more roleplay but please think about this first question as you read below. Am I giving them enough combat?)

Now under there they find the town vault and the Rogue decides to pick the lock. After the Paladin sees that he has gained entry into the vault and inside is a small safe that looks like the towns coffers he tried to stop him from stealing anything. The Rogue wasn't listening and keeps rolling for a lock picking check. So the Pissed Paladin rolls to attack and misses a kick he aimed at the rouge to make him finally stop.

Which he did once he saw that his Dad was ready to attack him and heres my second question. How can I prevent this sort of schism between the two players or leverage future events and maybe create learning situations? (Ok just a little more I have one more question)

(I paint the scene for the characters now) Futher down they go and start to notice cobwebs under their feet and they are reduced to half speed because of the webs. They come into a room filled with spider eggs sacks everywhere and on the ceiling are sleeping spiders. I roll a d4 and let them know that they have two rounds before the spiders awake. Paladin starts moving across the room but the Assassin wants to kill the spiders, the eggs and the rest of the siders. So he starts swinging at it all until the Paladin convinces him to come over and roll to lock pick the Elvish door. They get through on the second roll just ahead of the swarm of angry spiders.

So this is my last question is there another mechanism or something I should add to the game that makes it fun for both a 7 year old and a 32 year old? I want to provide a fun experience for both players but also something with substance. If rolling dice is what will keep a kid engaged and focused should I have him run another one of those NPCs to let them handle bigger encounters? Thanks for reading this all and for your thoughts and new ideas!!