What's the legal remedy for a betrayed Filipina working in Canada with
2 children still in the Philippines whose unfaithful husband is having
a mistress while working in Spain and about to file for Spanish
citizenship?

Your question is extremely complex, in fact, one of the most complex
legal questions I have seen here. To get a researcher to lock it,
you'll need to review the pricing guidelines:

How To Price Your Question
http://answers.google.com/answers/pricing.html

At the outset, you'll need a lawyer in the Philippines AND Canada,
assuming you want your children to end up with you.

Good luck to you, I'm a Mother, you must feel horrible.

~~Cynthia

Yes, this is a very complicated and unfortunate situation.

It is not clear what you would like to have answered:
Presumably your husband is a Filipino and thus eligible for Spanish
citizenship after tw0 years residency in the Spain.  To do this, he
must get a certificate of good conduct from his municipality.  A
Spanish lawyer should - or maybe a divorce expert in Canada could -
know if this can be upset by starting legal action against him  - for
divorce, support, child support.

http://www.graysworld.co.uk/spanish-property/spanish-citizenship.htm

I don't know, of course, which action you want to take, or which would
be most favorable to your interests.

Here is a website where you can find "pro bono" legal services in
Canada that could be a first help to you. They  - one of them -
probably will not be able to deal with the complications of your
situation, but should be able to suggest what choices of action you
have in Canada.

http://intraspec.ca/law_resources.php

It is also possible that a sympathic person at the Spanish Embassy or
a Consulate could be of assistance.  COULD be; if the person got the
impression that your husband was an undesirable type person to have as
a Spanish citizen.

http://www.embaspain.ca/

It would seem that any suggestions here will all point in the same direction:
you must get legal advice.

I am very sorry to hear about your problem,
Regards, Myoarin