Well, according to that Wikipedia page, people who used the term “race suicide” believed that “desirable” people were having too few kids and “undesirable” people were having too many kids.

So Interpretation A of this cartoon is the following:

1. the Landlord believes in the “race suicide” concept, and believes that Everett is an “undesirable” person who has too many kids. The Landlord is insincere when he says that Everett is “estimable” and a “desirable tenant”.
2. Everett scorns the “race suicide” concept, and argues for his kids’ right to exist.

And Interpretation B is:

1. the Landlord sincerely thinks Everett is “estimable” and a “desirable” tenant, but just doesn’t want his kids to live there for some reason
2. Everett believes in the “race suicide” concept, and is telling the Landlord that without his kids, “race suicide” would occur.

I’m leaning slightly towards Interpretation A because I think the Landlord is being insincere, but I could be convinced otherwise.

Edit: on second thought, I don’t think Interpretation B Point 1 relies on the Landlord believing Everett is “estimable” and a “desirable” tenant, only on the undesireability of the kids living there.