Child A is 18 and Child B 21.
Both children are adults and, in general, any arrangement whereby you claim to have made a "gift" to an adult, when the adult has no knowledge of it, and no access to or control over the money supposedly "gifted", is extremely suspect. I doubt that it would survive an audit; Revenue would take the view that there is no gift until the recipient is actually able to access the money.
If you want to claim the small gift exemption then you do actually have to make a gift. And the essence of a gift is giving; you have to give them the money. An inevitable consequence of giving them the money is that then it's their money, not yours. They decide what to do with it; you don't.
Except . . .
Child B has issues which means that we may not ever wish for him to be able to access the money himself and we would be thinking of a trust of some kind for Child B .
You don't say what the issues are and I get that in a public forum you may not want to go into detail, which is fine.
What you are talking about is a discretionary trust — one in which the trust property is to be used for the benefit of a person or a small group of people, but the trustees decide when, and in what way, and how much, and in what shares, etc.
In general, Irish tax law does not favour the establishment of discretionary trusts. In some countries discretionary trusts are used to shelter family wealth from tax; that doesn't work in Ireland. Without going into detail, if you put money into a discretionary trust in Ireland you will usually end up paying more tax, not less, than you would have done without the trust. So they are not much used.
(Also, relevantly to your query, in general putting money into a trust is not a disposal for CAT purposes; CAT arises when money comes
out of the trust, to the beneficiaries. So if you put €3k per year into a trust for (say) 10 years, and then the trustees decide to pay €30k plus accumulated earnings out to one of the beneficiaries, that's not 10 annual gifts of €3k; it's one gift of €30k-plus. So this isn't an effective way to access and utilise the annual small gift exemption.)
Having said all that, there are special rules affording more favourable treatment for a discretionary trust set up to meet the needs of a person with an incapacity. Obviously we don't know whether your child's issues would amount to a incapacity; they would need to be issues that mean the child is incapable of managing their own affairs. Plus, to qualify for relief, the trust would have to be used exclusively to meet "qualifying expenses" for the incapacitated child; treatment costs, maintenance costs, that kind of thing. The bottom line here is that, if you're looking at a discretionary trust for an incapacitated child, you will need professional advice that looks at your circumstances, and your child's, in more detail than is possible (or appropriate) on an anonymous internet forum. All we can really do is point out that a discretionary trust
can be a viable option in relation to a child who is unable to manage their own affairs and, if you if think it might be of benefit in the circumstances of your family, you should go and get professional advice.
The other bottom line is that the overheads of setting up and running a discretionary trust of this kind are probably disproportionate to the value of the trust, if all you're thinking of putting in is €3k or €6k per year for a number of years. They are more commonly used in connection with a will, where an incapacitated person is inheriting hundreds of thousands of euros.
The other, other bottom line is: don't let the tax tail wag the estate planning dog. These are your children; they can receive up to €400,000 each from you free of CAT. Plus, they are adults and perhaps are, or in a few years will be, earning salaries of their own. The annual small gift exemption on top of that is chickenfeed, frankly. If your child with issues has particular needs, focus on designing arrangements that will meet those needs and then worry about those arrangements might be tweaked to make them tax efficient. Do not start with the small gift exemption and then design arrangements with the primary purpose of utilising that. The small gift exemption is not the important thing here.