How much say should parents have over others using diminutives

I dont know about should but when it comes to oldsr chikdren and teenagers and their mates, the answer WILL BE zero.

Assuming the parents are in agreement (and so is the child if old enough to express an opinion), they do have a level of practical control. If, say, the creche manager or babysitter or maths tutor or piano teacher insists on calling Bobby Bertie after being asked not to do so, the parents can stop using their services. I'd be surprised if someone associated with the child's school insisted on using something other than the real name or "known as" name, and I'd expect the school to intervene if the parents complained. You can't stop random children and adults in the community from using the names they want, although if it's at school there may be cases where it's perceived as bullying and therefore managed. You can teach the child to speak up on their own behalf and correct someone making as mistake. I'd hope family members and friends would comply with the family's and child's wishes if asked, but some people are arseholes.

The parents should have 100% control. I think it's outrageously rude to call a baby a different name to what his parents have told you his name is. The only exception is if it's an even shorter version, for example if they're using Bobby and someone calls him Bob, I think they'll have to go with it. If you introduce your child as Bobby and someone randomly starts calling him Bertie, that's not on.

This is different to nicknames like Smoosh and Cuddle Monster etc. which are person dependent and a baby can have many different ones.

Surely as a baby you'd respect the name the parents chose and wouldn't use a diminutive - that comes older when the child has more awareness and their own preferences! Well, IMO anyway.

I think it also depends a bit on the name.

For example, if the child is Elizabeth, known as Lizzie, and family start calling her Beth, that is being a bit wilfully divergent.

But if Angus, known as Gus, gets Gussie from Grandma, that falls more within natural affection.

I have known parents to say to school teachers/nursery "We don't call x such and such diminutive" and I think if the dc hasn't expressed a desire to be called that, then that is well within the parents' rights. I would say the same applies with parents of friends of the child.

Once the children get old enough to have an opinion themselves - and actually I think that varies with the the child as some really don't care and others do - then the child's opinion necessarily becomes relevant even if that goes against the parents. So if the teacher calls dc Robert Bobby and Robert says he likes it, that starts to be just bad luck for mum I think!

Family members of a baby are a bit trickier, simply because these names are born out of affection, and their levels of affection are likely to be stronger and more genuine than caregivers or teachers. But i do think it is a bit naughty if family know mum and dad don't like a particular diminutive and they fall into using it regardless. If it arises more organically, I'd be inclined to let it go - and just try not to cross-pollinate it to school!

But if your parents start antagonistically calling your baby Beth despite you telling them not to… what for exactly? Why would a person do that?

It’s a horrible power play for other people to refer to a baby contrary to the parents’ wishes.

No guarantees but it wouldn't stop me using it, but I wouldn't call a baby Matthew if I hated Matt.

Yes, basically. If you hated Beth then don’t choose a name that it is a very obvious nickname for.
If you don’t use it and call her Lizzie instead for example then is unlikely that she’ll end up being a Beth but there are no guarantees!

If it’s a baby I don’t know why you wouldn’t just call the kid the name the parents are asking for, or not call them a name the parents have asked you not to call them. Seems massively disrespectful and like you are trying to cause trouble.
assuming there’s not a massive drip feed like the grandparents have got 5 grandkids with the same first name.

if its an older child who is requesting a name then no the parents don’t really have any say.

If the child is young, then I believe the parents should insist on their preferred name.
For example, this is Robert, Bobby for short should mean that.
When Bobby's a teenager or adult he can choose.

@Henriettina , I like Elizabeth but not Beth. Does that mean I shouldn't use Elizabeth?

They shouldn’t have chosen a name if they dislike an obvious diminutive.

But equally, it’s very rude of the family members to keep using it.

In the end it’s not a battle the parents will win if the child isn’t onside. I have a 3yo who I assumed we’d use a diminutive for, but she’s totally insistent on her full name.

I think you can when a child is still young - it's pretty rude of other people to disregard the parents' choice and to call their child something other than what the parents have chosen.
My grandparents did this - used an abbreviation that my mum really disliked and had asked them not to use. My mum ended up giving me a completely different name!

Obviously as kids grow, nicknames develop amongst their friends and grandparents etc develop pet names, but on the whole, I think it's just good manners to call a child by the name the parents prefer.

It depends on the age of the child. A baby then yes you can ask for a different nickname to be used but as a child gets older then none at all. Though no doubt a child will choose what they like. Over 25 years ago I used to watch a young boy after school called Michael who was then age 6. I would also sometimes watch two of Michael’s friends Isaac and Ellen along with Ellen’s little brother Robert. By the time they had reached 10 (Robert) and (12) the other three they were known as Mike, Zac, Lennie and Robbie.

Lay down the law and take no shit!

None

You can’t control this - nicknames are organic signs of affection

None at all.