It's normal that toddlers are totally thoughtless and trash the joint, isn't it?

[thing1andthing2]

DD is 2.9y. I have a 14 week old DS. Every time I turn my back on DD for a microsecond she is busy testing everything to their limits, pulling things apart, tearing up books, pouring water all over the floor, crumbling her snacks to dust and rubbing them in the sofa, hiding parts of her toys never to be seen again so everything has parts missing etc etc.
It's doing my head in, especially with the rain and being stuck indoors. In September when it was dry we just went to the park everyday. It is normal though isn't it? I don't want to yell at her about it if she really can't help it.
My psychologist friend says its something about their frontal lobes not being developed properly yet and not being able to remember instructions and control impulses .
I'm going to go and put both of them in the bath this afternoon just for something different to do .

How on earth is she going to learn not to break things and make a mess if you don't tell her off?

Tbh, that sounds like an unreasonable level of destruction if it's happening as often as your OP suggests.

At almost 3 mine would have gotten a telling off for purposefully damaging a book or furniture.

I'm not the tidiest person (understatement) but I now realise that the house was near perfect pre child. DD is 2.4 and the last few weeks we have just gone craft mad. Bought some Mister Maker sets and little one loves it. Also did an afternoon of painting. I found that as I had embraced potential mess before it happens made it easier to deal with!

Its normal to get up to mischief, yes. But you need to tell her off all the same - she won't know its not acceptible otherwise. She may still do it, but she will understand its wrong. DS2 is punished with naughty spot (timeout, whatever) when he is willfully destructive.

I think at 2.9 they are mostly capable of controlling themselves. I expect DS (who is the same age) not to make a mess with food or with his toys. He knows that if he makes a mess deliberately or otherwise he has to help to clear it up and this seems to cut down on the "accidents" a lot! I model it myself as well, so if I spill something I clear it up, and sometimes ask him to help me just as I will help him when he has made a mess.

That said, they are a bit little to keep track of belongings so I am quite careful with puzzles etc and I mostly tidy them away or bits get lost.

And invest in waterproofs! Seriously! Waterproofs, wellies, warm clothes and at least 2 hours of madcap running around seriously reduces the energy left for destruction

We have a fair few things broken by our 3 and 2 year olds. On the other hand, they really get told off if they break things, especially anything of value and even the 23 month old knows some things are off limits. If there are no consequences for behaviour (e.g losing a toy if they deliberately damage it or throw it down the stairs) why on earth would they know any differently at any age? Also, the idea that they have a right to break things or make a mess breeds a real sense of entitlement. If they make a mess, they have to help clear it up or there is a more serious consequence.

Most people in the world cannot afford for their children to destroy their possessions, so they make sure that they don't.

She won't retain the information if she's not given it to process

Tell her no. Then tell her again. Keep going till she learns, nothing wrong with a bit of rote memorisation!

Oh don't worry I do tell her off, I just mean I am tying not to lose my rag with her. You can tell off without yelling, right? I mean, I don't want to shout and steam and drag her by the arm (which I feel like doing when she's done something again, but don't).
And I'm constantly asking where things are and trying to keep the toys together in the right boxes etc. but my DH doesn't seem to think its a big deal if half the toys get lost and go away in the wrong place so I feel I'm fighting a losing battle half the time with both of them not caring where things go.
I'm still really struggling with managing two children especially a baby who wants feeding all the time. This afternoon we sat down on the sofa, me to feed the baby, her in front of bob the builder on the laptop with a cereal bar. Next time I looked the cereal bar was in crumbs all over the sofa. I was so cross, so she got no more bob the builder and no more snacks. Told in no uncertain terms why. Not sure what more I can do?

I guess I feel she's a bit "away with the fairies" half the time and I'm not sure this is normal for this age.
Generally if she's on the way to breaking something by "exploring its properties" I take it away and explain why, especially if I can predict it based on past behaviour with same or similar items. But with the baby I can't keep a close eye on her every second.
We do loads of craft etc, she loves that. But again, painting etc doesn't work well unless I'm in arms reach and I can't be all the time as I'm feeding the baby half the time. Seriously I have a super hungry baby.
I paint her as a little devil but she's an amazingly sweet girl, mostly delightful. Just in her own little world.

For toys, I think that, unless they are being deliberately destructive, they are their toys and, ultimately can do what they like with the natural consequence of losing the toys if they break them (which is amazingly hard with modern plastic). If she is damaging your things, it is another matter. Realistically things will get broken with toddlers and, to some extent, you have to live with it. On the other hand, they should not be getting the message that they can break things with impunity. Break a toy, no more toy. Damage your sofa with crumbs, no more eating on the sofa etc.

I really don't think that young children should be allowed to eat near a laptop, or even on a sofa, really (although there are exceptions). Ours always have to eat in the kitchen/dining room except for "movie night" one night a week where they are allowed to sit on the living room floor and eat an easy dinner in front of a film.

In my opinion (although my wife does not necessarily agree) messy play places exist so you don't have to destroy your own house.