Leaving aside for a moment the recording (which in itself can create complex legal debate over coaching & admissibility as evidence).... If you genuinely suspect (serious) psychological harm may be occurring, then there are a bunch of reasons why YOU should NOT be talking about it with them & most especially questioning them about it...
If something WERE happening, the kids testimony is often all there is. Are you aware that questioning children can contaminate a kids later testimony? That is why these things need to be left to trained child psychologists, who are supposed to take great care to extract information that can be relied on.... Kids are not like adults, they don't think or process things like an adult, & what you think may be a logical extension of something said could be light years from what they are actually thinking or meant... They are also hard wired to tell an adult what they think that adult wants to hear.... Repeated questioning on the same or similar topic for eg, can result in a kid assuming the first answer must be wrong, & they may subsequently change the story slightly ..... Add to that the questioners own confirmation bias, ie, you assume something, then (sometimes even unconsciously) note anything said that confirms your assumption.
So what to do ? ..... IF you suspect SERIOUS psychological harm (because it shouldn't be a news flash to anybody that dealing with separation, not seeing one parent nearly as often as they may like, & coping with separated parents is in itself psychologically challenging for kids & has consequences) then if a kid says or does something that concerns you, WRITE IT DOWN... Don't question them about it, don't record it. Those notes can then later be handed to a child psychologist if it ever comes to that... They are the ones trained in this stuff