There are two things you need to keep in mind:

1) even by the time the addition is finished, they may no longer be interested in bathing with each other.

2) you may need a separate or point of service water heater just for that bathtub.

why a 6' tub? is someone going to lay prone in it?

A lot of people put in 6' tubs lengthwise, but the width is what is going to make this tough to fill with enough hot water to use it a lot. I think a 6' conventional width tub would be big enough for multiple bathers when they are small kids and then conventional enough in size that it would get some use later on.

Most people who have these extra large tubs apparently use them very little after the first year or so...there have actually been industry surveys regarding this.

That is a whole lot of tub! Here are a couple of things I'd think about...

There is an age range when bathing together is fun and appropriate (not to mention, great for photo ops!) but all too soon they grow out of that phase so I'd ask if myself if I'd want to have to fill an oversize tub for individual bathing.

Will this be a tub/shower combo? If it is, I wonder if 22" is tall for kids climbing in and out to take a shower, once they are showering instead of taking tub baths.

We went with an MTI Harmony tub and it's been installed, tested, but not yet used as our masterbath reno is still weeks (months??) from completion. We found our best price at tubzlady.com (including free shipping!) and received excellent advice from Koren with all of our many questions. (I have no affiliation with the website, just recommending a resource!)

Good luck with your project!

I just took another look at the specs for my tub (71" x 41" x 24"). It's pretty huge, plenty big for my DH and I to both enjoy the whirlpool and airbath. I can't imagine filling it up for daily baths though.

Treasure - I compared your specs to my specs: the tub I'm looking at is basically a foot wider than the tub you have, but my tub isn't quite as deep. I guess since it's a virtual swimming pool for my 5, 3, and 0 year olds I think the extra width in our case is needed.

I'm not going to use it as a shower. I'm aware it might not have enough hot water, but I have setup an infinite amount of hot water even with the GPM of a tub filler.

That's a separate question - are all tub fillers created equally in terms of GPM?

Thanks for all the advice.

I think the posters above had made some good points about the kids, however, I will say those moments are precious & the kids have a blast--besides, what better way to be the exception to those research surveys than increasing tub use with the kids taking a swim? Our 7 yr old loves to take a swim in our tub as well as his grandparents' tub. Mom and Dad have the BainUltra Meridian in the Ellipse 7240 style--perfect for multiple little bathers taking a dip together & nice for a couple's or single bather bath experience. It easily accommodates taller bathers too at that size. I know Mom has had all 3 grandsons in there at one time when they were a little younger with no problems. It's also available in a rectangle style with end drain or right/left center drain options.

Here is a link that might be useful: Bain Ultra Meridian Tubs

I HAVE good resource where you can find every type bath product. use ibathroomsuites to buy bath product.

Your money would be better served to create an actual swimming pool rather than a big tub. In a year's time, the kids will be doing more fighting together than bathing together and you'll be refereeing because one of them "looked" at the other. They certainly won't want to actually risk "touching" one another---especially naked! EWWWWWWW! GROSSS!

We put a Kohler 6' Tea for Two tub in with our bath remodel 5ish years ago and my 3 year old twins have plenty of room to play together in it, and there would be space for more. The other benefit is that because it's quite deep, it minimizes how wet the rest of the bathroom gets. It's also a great soaking tub for adults and isn't too deep to be used as a shower (we've been using that as our shower while our master is being redone).