"MIL kept making comments at DD, telling her she’s too old for nappies, dummies etc. and she’s gonna get bullied when she starts school in September."
What am I reading? Sod your MIL being right...She absolutely should not be speaking to your DD like that. I would be ( have been) furious at similar comments...
How dare she? Its one thing to tell you... it's quite another to hurl undermining criticism at a four year old.
All potty training issues aside.
"You're too old for nappies and dummies" - everyone thinks you are acting like a stupid little baby.
"You're going to get bullied when you start school" - School is a horrible place where people won't treat you well. You will be there soon. Why the hell is she putting that idea into your DD's head, especially as you have a newborn on the way?
This is horribly unkind. It's making the whole potty training thing into a much bigger deal by raising your DD's anxiety and making life harder for you too.
My MIL used to make similar comments about ours... guess what.. they learned to use the potty and were fine when they went to school. She also went on about all her GC being "too fat"... until SIL flipped and read (well roared actually) the riot act and asked MIL if she was trying to spark eating disorders (thankfully saving me the job!) MIL also asked my middle DC who plays team sports after work twice a week, regularly trains at a gym and runs marathons, "are you on a diet? ...You should be!" Yes, They are not as skinny as they were at 16 because they are now a fully grown adult.
Having them not being stressed about potty training was a big factor. If they had an accident or two on the way.. so what, they got the hang of it. And I agree with you OP, it is easier now the weather's warmer and they are in lighter clothes. The pull up nappy/pants were a godsend for nighttime in the early days or on long car journey's but the key thing was to be very matter of fact and non critical about it. Its just a normal part of life.
Tell your MIL to stop making these stupid comments at your little DC. You all know her opinion now.. she doesn't need to keep repeating it. She can say it to you if she absolutely has to but not in your DD's hearing.
I suspect she is doing it as a way of indirectly criticising you..(the thinking is... if she's talking to DC you are hearing it and will feel guilty) I bet she hasn't taken your DH to one side and told him to speed up the training. Call her out on it and both of you stand up for your DD whenever she makes comments like that... "MIL I don't agree with you and please do not speak to DD like that"
It's particularly important that people don't call her a baby when you have a new one on the way. You are going to need to bolster her confidence. Don't let your MIL start comparing her to the baby when they arrive.