Honestly, I have no idea. The one thing I know about 17th and 18th century noble child rearing that was damaging to small children doesn't apply to this case, because the kid was living with Liselotte in the same household if she could see him in the evening. Because what a lot of nobles and rich burghers who wanted to be elevated to nobility did was basically hand over the baby to the wetnurse (often a farmer's wife) to rear him or her in the countryside for the next three years. Now, the problem here was that in a country where the population is steadily getting poorer, the wetnurses get tempted to accept not just one baby but several in order to make more money, and sometimes even hire even more poor sub wetnurses. (And let's not forget, all these women have their own babies, too, that's why they can be a wetnurse!) So there isn't enough milk for any of the babies, who get fed with water to make up for the missing milk instead. => lots of dead children. But like I said, this might apply if you're a noblewoman, a Countess de This and That, who is lucky if she can get a room in overcrowded Versailles, not if you're Madame with your own royal residence Saint-Cloud and with the wetnurse being directly under your and your staff's supervision. There's an earlier letter from 1774 when Liselotte is pregnant with child No.2, the future Regent, and talks about child No.1, the doomed oldest boy (at this point not yet ill, where she describes ihm in a way that shows she sees a lot of him, so even if there is a wetnurse and a governess and maids and what not, she clearly was in the nursery a lot:

Regarding my little Rauschenblattenknechtchen - this is a German Baroque term I've never come across before but which she uses for babies occasionally, literally "Whisperyleavesservant", which makes no sense - he deserves the name by deed and is a terrible savage, laughs a lot and wants to be carried from one place to the next, will only now after Easter get a frock because in this country, they have the children wrapped up an awfully long time; then we will see whether he'll learn to walk soon or whether he won't. He doesn't have a single tooth yet. Please, write to me how much Milady-Kent-Powder one could give him if, which God may prevent, he has a fever once he teethes? How much is too much? Because he's less than a year. Thanks be to God, he is a cheerful child, but can't do much yet but piss the way the eleventh Prince ( one of her Hannover cousins) did when he was that age.
Enough now of him. Regarding the other little madcap, who is halfway on his way by now, he's causing me a lot of bother, for I'm sick as a dog every day after the meals, for two full hours.

"Wrapped up" vs frock I take to refer to diapers vs clothing, only what was done back then weren't diapers in the way we know them, but the babies being literally tightly wrapped up:




You can see why doing this for up to a year didn't strike Liselotte as very healthy.

Re: weaning in particular, though, like I said, I don't know something that could apply in Liselotte's circumstance. I don't know what Milady-Kent-Powder is, either.