buying a house in US with TN visa

My husband has a TN visa working in TX, he is going to renew it in two months(3rd time). I will move with our children to join him soon. He just signed an offer to purchase a house in TX days ago, I am a little worried about the renewal of the visa if the officer challenges why we need to buy a house in US. By the way, we still have a house in Canada, does this make us in a better position when we are asked where is our permanent residency?

Yes it does although it's not required, but the more ties you have to Canada the better it sounds, and a house is a pretty significant tie. Just make sure his offer letter says three years as the maximum validity of TN-1 was extended to three years a couple of weeks ago.

Bear in mind on TN-1 (and all non-immigrant categories) is that life can be complex for you and especially your kids, as although they can go to school, neither you or they can work (even in a volunteer position, if it would be something that normally would be paid). This causes problems when they grow up. Also, on TN-1 even if you live in Texas they have to pay foreign tuition costs when and if they go to university.

There are some non-immigrant categories (primarily E and L) that allow spouses to work, but not children.

The point I'm making here is that although you may have it mind that it's more permanent than you're letting on to USCIS, in reality the law makes it temporary, so there's no point pretending really, unless you're 100% positive the employer is going to sponsor your husband for permanent residency.

if the children complete high school in Texas and have put in 3 consecutive years prior to Univeristy they are allowed to pay in state tuition