This is a concern I had too. XH is not in 1&2s life, at all, so if something happened to me I wanted to make sure that they would stay with DH or at worst, go to one of my family. XH can't be trusted.

Anyway, I stopped worrying a couple of years back after reading a story in the newspaper. Borrowed this from a site

Interestingly in Australian law, custody is not solely the domain of the the biological parents of a child. In Australia it is possible for the step-parents, grandparents, elder brothers or sisters and in fact any adult who is concerned for the welfare of the child to apply for the right to care for a child. The is a result of the the FAMILY LAW ACT 1975 – SECT 64C which states that:
“A parenting order in relation to a child may be made in favour of a parent of the child or some other person.”
The reason it was in the paper was because it had just been tried in court and succeeded. The step-father was given legal custody of his stepson over the biological parent that had abandoned him as a baby.

Your chat with legal aid should help sort this out in your head and yes a will.