Backstory: my mum has two grandchildren, both from my brother. She knows DP and I are thinking about babies (though we're in the early stages of TTC, which I suspect she doesn't realise). Mum has always 'let off steam' to me about DB - and particularly about his wife. She likes his wife very much, but she has a blind spot with my brother - he's the golden child - and in particular, she and my dad are united in the firm belief that he is the conscientious, strict parent while my SIL is the lax, permissive one who occasionally undermines his good order.
This would be funny if it weren't so annoying, because everyone else notices in five minutes that my DB's kids have him wrapped around their little fingers and that SIL bombs around getting things organised. She's also developed the belief she is sadly denied opportunities to see her grandchildren, because they often go on holiday to visit their other grandparents in Spain (as you would). DM usually admits she hadn't actually invited DB and family, or suggested she visit them, during these holidays, mind. 
Increasingly, it is clear DM is using comments on this situation as a covert way of hinting about what DP and I should do, should we ever have babies. It is very obvious this will put her in direct conflict with DP's mum. DM is a very 70s, conscientious-middle-class-parenting type, thinks breastfeeding is practically a sacrament, that babies should never be in nursery and should always have a large garden to run in, and should always be stimulated with naice books, educational trips, and no TV. DMIL proudly weaned hers before one month, is very relaxed about TV, etc.
Both are vocal about expectations, though DMIL is markedly less annoying because she simply says what she thinks, while DM prefers to convey her views through the medium of contorted facial expressions and hints so subtle DP generally has no clue what she's getting at.
They have never met. They will meet in the next few months. DP and I have realised that this is likely to result in a, erm, robust exchange of views.
Should we a) warn them, trusting them to act as adults, b) attempt to mediate, pointing out that as the potential baby's potential parents, we might have some say in issues like breastfeeding, or c) light the blue touch paper and stand well clear?