Not strictly a wwyd, more of a how would you do it.
MIL is very kindly helping us out with childcare once a week now I'm back at work. She is a doting grandmother and absolutely adores our DS so is really enjoying spending time with him and it's a great help to us. However, the last 3 times he has spent the day with her, DS has come back with nappy rash. He doesn't usually suffer with nappy rash (only ever had it after spending the day with her) and generally his skin doesn't seem overly sensitive. I provide wipes, cream and nappies so I don't think it's resulting from any different products she could be using. Today I made note of me number of nappies I sent with him - only one was used (I sent him with 8!) even though he was with her from 7am until 5:30pm, so it's fairly clear she isn't changing him often enough. As this is now a regular arrangement, we obviously need to address it.
MIL is a lovely lady, but in my opinion can be a bit prone to taking things too personally or being a little dramatic. We generally get along well, but my feeling is that she will feel attacked/criticised if we put it as simply as above. How would you broach the subject so as to cause minimal upset but ensure the message is delivered effectively? I don't want to upset her at all, as we are very grateful for her help - but equally I'm not happy about DS being left in dirty nappies all day!