I must advise you against doing this will yourself. The reason is that it would then be extremely easy for the children to say that you had put your partner under some kind of influence.
They can contest the will in circumstances like this on two grounds, undue influence where you persuaded him to do something which he would not otherwise have done or under the Inheritance (Provision for Family and Dependents) act if the deceased person doesn’t make provision for dependents children and family in their will.
If you speak to virtually any solicitor, they will do a home or hospital visit (for which there is an extra cost but would be worth it) which would thwart any claim for undue influence.
If your partner does not write a will, the state passes under the rules of intestacy which means that you would get nothing. For that reason, it is essential that he writes a will.
If he leaves half of his estate to you and half to his children, it is simply not worth them contesting that because, with an estate of this size it is probably going to cost them more to contest it (with the risk that they would lose) than they are arguing over.
Can I clarify anything for you?