The following quoted text is about using first and zero conditional constructions in the F2F ELL book. Please, find my question below the second paragraph.
Our children get their pocket money every week on Saturday. Of course we ask them to help at home.
In the following paragraph, the adverb usually suggests using zero conditionals which is used for situations that are always (in fact, almost always!) true, like habits and rules. There is not problem with this - it's clear. The issue (if you will) is with the last sentence. I understand it's in the unlikely situation, "they'll their children". But then this unlikely situation is also a rule! The parent is not talking about a specific situation in time (in real life), but a hypothetical case while he/she is talking about rules in their house. The question is: which is more logical (if both are perfectly fine in the context -or are they?):
If there isn't one, (we 'll tell them) that we're disappointed.
If there isn't one, (we tell them) that we're disappointed.
And usually they do everything we ask them to do. And if they don't do it, there is usually a good reason. If there isn't one, (we'll tell them) that we're disappointed.
The following is the end of the text:
We strongly believe that this is a much better idea than the threat of no pocket money.
I think the reasoning is that it comes as a contrast to the "usual" case of there being a good reason which qualify for a zero conditional. Hence, the exception qualifies a first conditional (The usual rule is a back ground to the exception). However, if it were to come up in a non exception to a mentioned rule, then it would be a rule alone of something rare in which case it qualifies a zero conditional.
I would love to confirm this from credible sources, but knowledgeable and native speakers' opinions are very welcome.