well, i think you were trying to be fair, and generous, but maybe due to your own history you have gone about it the wrong way.
it seems you like to be ultra secretive, and do not trust neutral professionals, eg solicitors.
you should have consulted a solicitor or financial adviser as to how to do what you wanted re gifting the money to DC.
but you are obsessed with no one knowing anything about your assets. this has clouded your judgment. and in a way you expected too much of your DC, to keep a strict watch on what/ how much they said to their partners and others re it.
of course people will speak to their partners, that is inevitable.
and it is very hard to speak to a partner, and say mother will give me £100; but no, i must not discuss whether she actually has £1000, or what she will give my siblings; i must not speak as to that.
obviously they ought not to have mentioned anything about money to other people, but knowing them as you do, their way of life, habits, company kept etc, you could have predicted this might happen. so that was a lack of jugdment, or caution.
if they are living lives you disapprove of, then givingg them money was always going to be a mixed blessing, at least.
if they/their partners are taking drugs/alcohol, living rackety lives, how could you expect them to behave with discretion.
you say SIL lacked manners in asking for more. why does this surprise you. it is predictable. lack of manners is the least of it.
also you could have taken professional advice without revealing all your assets. you could have limited their advice to one bank account, and moved money to have enough in that account for what you proposed to gift to DC.
you have got yourself into a muddle.
don't give any more. be careful to make a will, where those parties will not benefit from your demise, and tell them that.
some people will do anything for money, esp if their brains are drug-addled. normal morality goes out the window.
it may be an unlikely scenario, but it does happen, and i would not be taking any chances if were you.
and do consult a good solicitor.