It sounds like the kids aren't ready for serious paddling and you need all four in one canoe. If so, the SRQ 18.5' would be my choice. Let the kids do as much paddling as possible. Also, some pre-trip practice is a good idea. 

I concur on the SR Q18, but maybe buy used from an outfitter - mine refinishes them to sell - and in 4 or 5 or more years you can probably seel it for not much less than you paid (and buy a pair of Q17s!)

Fallguy, Thanks for the info too! I'm not familiar with the Wilderness. How did it handle compared to say the SRQ18? Haven't seen a lot of them for sale or read a lot of reviews on them.

I've gone with 3 adults twice. Rented a Minn3, then next year rented a Seneca. The Minn3  was better, but neither was fun. Anyway you go, finding room for packs and people is tough. I tried to find a Minn4 to rent, but couldn't find one. I thought it would work best for 3 people after removing one seat. "Never again" is my best answer.
Renting is expensive, but I wouldn't want to own one either. I see the need, but hopefully the kids could handle their own boat by their early teens. I've seen several scout groups traveling with 13-14 year olds handling their own boat. You might have to be careful about your travel days, i.e. weather & distances, but I'll bet the everyone would have more fun.
Personally I wouldn't mind carrying both boats with light packs, if everyone could handle the rest of the gear.

Thanks for the run down on those boats Jim J Solo. Everyone has given me really great perspectives so far. I'm mulling a lot of this over because there are obviously some trade-offs to consider.

My first inclination was to go with the SR18.5 because it might give us the room for my son to take the spring creek seat and my daughter to sit on one of the portage packs behind my wife in the bow. At least everyone could have their feet touching the bottom of the canoe!

A Northwind 18 (really 18'9") for example, gives a little more room for somewhere around 42 to 44lbs for the ultralight. I've paddled one and seems fine in a tandem but I don't how it would handle with all four of us.

We've never paddled the Seneca or MNII or III (or IV for that matter) so I have nothing to compare those to -- some have recommended the Seneca but both that boat and the SR18 run closer to 55# and I prefer a lighter load 

Just to overly complicate this, I was thinking that one of the longer boats, out for an average 3 day-trip 3 x per summer (what we usually do) would probably be about 500 or so. Much cheaper than laying out for one of those boats now. Also if we decide to do the 2-boat plan next year or year after, we'd still be out less than the cost of a longer boat....hmmm. Lot's to consider here but thanks to you and everyone for the suggestions!

By the way, Happy New Year everybody!