Four things you can do besides tweak encounter difficulty
- Emphasize exploration and social interactions during the first few sessions. Make combat a bit more rare (ignore the 6-8 encounters per adventure day guidance in the Basic Rules, you are playing with beginners). Some of the encounters don't need combat. Coaching/nudging non combat solutions to some of the encounters with NPCs can make for some fun play.
- Equip each character with a free healing potion from an NPC before starting out. (That's more or less an insurance policy).
- Roll diceCall for die rolls only when you have to.
Combat calls for quite a few die rolls, and some ability checks in the published adventure need to be rolled, like picking the locks on locked chests. - Err in favor of the players.
That's how I started my kids out about 20 years ago with Basic D&D (25th anniversary edition/big box). My wife only played a few sessions (oddly enough, also a cleric) but she also got a benefit from that approach.
Focus on "How to Play"
- DM describes situation
- Players tell you what they intend to do Have them tell you, {and then, if there's something on the character sheet that is a good fit, coach them to and through it}.
- DM narrates the outcome
For tweaking the combat encounters: use the free Basic Rules (starting on Page 165) from WoTC to adjust encounters to Medium/Easy range at first.
Chapter 7 "Help" with Ability Checks
The linked rules paragraph is entitled "Working Together."
For ability checks: encourage the players to narrate a help action when one of the other two attempts an ability check. This does two things.
It encourages team work as a mind set,
it also offers advantage to an ability check which increases chances of success (roll 2d20, pick the better score)
More successes more often, especially with kids, is a good way to sustain engagement. This I've found to be a bonus even with experienced players for this edition. Get them to play as a team; the Help action rewards teamwork mechanically.
I would suggest getting a close friend or relative to play the Fighter, but you note in a comment that there isn't such a player available. If that changes, I'd still recommend it.
Option if it becomes available: "Phone a Friend"
Why? The Action economy in this edition of the game starts to get a little wonky when there are fewer PCs. When in doubt, default to one less monster/NPC, since the players as a team are getting 25% fewer actions or choices, when combat comes in the situation where there are 3 rather than 4 PCs.