TD renewal

I renewed my TN this week. I have 2 children with TD visas that expire in the next week. Is there a way for me to renew these TD's without travelling back to Canada or paying the $300.00 fee to do it by mail?
Can I go to US Customs at an International airport where I live and get them renewed?

No. TD is an entry status. They need to be entering US (so going to airport won't work), or you need to file I-539 before TD expires.

You only need to pay one I-539 fee for both.

Would it be a problem to renew the TD within 30 days of expiry? spring break will mean a return to Canada, that will mean 25 days after expiry.
Will this cause a problem?

With the current mood of CBP, I would be filing I-539 to remain in pending status, and getting TD when crossing the border.

It is never a good idea to willfully go out of status.

But it is your decision. TD is not a work status, so there is some forgiveness. However, neither is tourist status, and there have been horror stories.

Update, I went the I 539 route and I received a new I 29 within 3 weeks. Totally amazing.