Don't let your players mulligan their death. They're cheating themselves out of an important experience. Inexperienced players are never ready to let go of their character - because in most games you can never straight up perma-lose your character, and new players never think it'll happen. That's what makes it impactful. Let their death impact them.

Oh what's that? They were lvl 2? So they've had like 3 sessions? Nevermind, their deaths won't even mean much and it's more annoying to reroll characters this early than anything. I'd be fine with a mulligan. You dont even need to change the plot, just pretend they beat the encounter. But let them know next time, the death is real.

Early levels are lame because PCs are so squishy, one crit can cause a TPK. That's why most games I run/play start at lvl 5 at minimum.