Southwest has a good solution for this. They prioritize boarding by the age of the baby. They were announcing "...for families with babies who do not yet walk..." That's a good criteria.
The other annoying aspect of preboarding were the families who were simply disorganized. I've seen them gathering up their things and dragging to the aircraft, with simmering fellow passengers all around them eager to get themselves on board too...
That's why I recommend at least trying what I used to do. I would stand up in the front, with everyone and everything ready to go and often I was simply gestured to go with the UM's and handicapped passengers. I'm not sure if I had less than three small kids and another parent whether that would have worked as well lol!
Wouldn't it be completely valid for a family with a small baby(ies) to be informed that preboarding will take place and that they must be ready at that time or miss out? Also, having a cut-off, say age 3, would work. I'm almost always alone when I fly with three closely-spaced children and honestly, I really didn't need to preboard, even with a car seat, once the littlest was around 2 1/2-3.
The only sticky issue is with open seating and on my last flight, I coughed up the silly 3 Euro fee for priority boarding because I simply can't herd three kids on that quickly and secure seats together in the mélé. This is an option open to anyone so no criteria needed.
If you are concerned, have you considered just boarding last? That way, you aren't blocking the aisles or getting in anyone's way.
I really discourage this because as a Flight Attendant, I had this happen too many times. If you get on last and there is a seating problem, you have given yourselves NO time to sort it out. What I can recommend, a luxury I never have is when two parents are flying, send one with most of the gear on first. That parent can make sure everything's okay and then the other parent trots on board with just the kids at the end.
But someone here gave the most excellent tip of not separating any diapered children from the diaper bag, even if it's only going to be for a few minutes. I wish I could give credit where due but the poor poster had a very unfortunate experience and amusingly recounted this, to save anyone else from making this very-easy-to-make mistake lol!