This is really something you need to discuss with the solicitor who is drawing up your wills. A financial adviser or financial planner will have a working knowledge of inheritance tax (IHT) and trusts, but the solicitor is the one who can help you set out your requirements properly and legally in the will.
Make sure you choose one who is familiar with trusts rather than just straightforward wills. They will be able to advise whether you should have a separate trust for the first child you mention, or whether one trust may be better with all the children as potential beneficiaries.
There are special tax breaks for trusts where one of the beneficiaries is a vulnerable person, which may be the case with your child. Again, the solicitor will be best placed to advise whether or not she would be eligible. If she is regarded as a vulnerable beneficiary, the trustees can claim special treatment for income tax and capital gains tax – provided the trust assets can only be used for her benefit.
While I appreciate your wish to avoid unnecessary costs, it is far more important to ensure that it is all set up properly to ensure your assets will be distributed in accordance with your wishes. If not, it could be both costly and distressing for your personal representatives to sort out.
The solicitor should be able to give you an approximate cost before you commit to it, including any fixed costs and an estimate of any time charges.
While sorting out the will, you should also use the opportunity to arrange lasting power of attorney if you have not done so already.