There is a huge misconception about how SBMM work and how K/D work, just to make something very clear: 0.6 K/D is the avg on players who did not quit to protect K/D in the past (yes its been patched now so you cant protect your K/D any longer). Game looks at your recent performance, not counting what you did a year ago or something.
When you make a premade with 3 people you will matchmake using the person with the highest MMR + they try match you with other premades. Most of these are proper teams playing ranked together too so you will feel the lobby being much harder then when you solo. On top of this many teams mostly play ranked and when they play pubs they just fool around and keep dying more there to silly things then ranked, and hence get lower SBMM for pubs. I for me hardly every try winning a pubs, I just want to have fun and play with weapons I normally never ever pick up. Hence I matchmake with lower lobbies in pubs then I do in ranked. Also if one of your lower levels in the group had an amazing game before you group with them you will get some nasty matches, remember its easier to get a proper awesome game in low SBMM lobbies 😉
For instance when my son and daughter plays Apex I normally end up watching them and cheering them on even if they have higher K/D then me, because if they come to my lobbies they dont have fun at all (they are 25 and 45, I am 500), I keep dropping silly in pubs just to practice chaos management or get a proper laugh and that stuff is NOT healthy to your K/D but still my lobbies are not pretty. While their lobbies they can stop and fix things without getting punished, and you can do a multitude of mistakes without dying because their lobbies are at their level as it should be. If you guys play together you should mentally prepare and have a strategy when you jump, its possible to enjoy if they are prepared for what they will meet for sure and you can play safer so they have a chance to catch up. Normally the stuff a low level struggles most with is finding their way, rotating properly, game sense and looting correctly and fast, taking IGL and fixing the rotation for them and doing it in a way where you can control how fast the game is they can enjoy this a whole lot as they can focus on 1 or 2 things instead of everything.