I would certainly give it a look, if you think you could manage financially, but if it is likely to set you back at all in terms of financial security or job security, I'd probably advise against it.
You'd have to look into the waiting periods for private health insurance with pre-existing conditions, because any costs will have to be covered by you, until those are over. You also need to consider costs for the 1:1 cover at school, and whether that would even be possible. There is a very good home school network in Australia (due to there being many kids who live on cattle stations, hundreds of kilometres from anywhere), so I wonder if that might be an option for you? Could you manage with just one wage coming in, to give your son 1:1 at home?
What about job security for you coming back to the UK? We were lucky because OH is at the top of his profession so it was easy for him to find work when we moved back, but I know others who have struggled. Would your DH go back into the same role that he is in now? What about you?
What about relocation costs both ways? Would the employer cover those? Flights, shipping, setting up a new home etc is all hugely expensive, and you will have those costs twice in three years. Even things you'd not think of, like going back to having no no claims bonus on car insurance, and building up a store cupboard of ingredients and equipment in the kitchen can add quite a bit to the expenses.
Would you sell your house or rent it out? What are the likely costs, pros and cons with each of those things? Do you have pets? If so, would you re-home them, ask someone to look after them or fly them out? What about your furniture and other belongings? Would you bring those, leave them in storage, leave them in your house if it is rented? What will you do at the other end? It is rare to find furnished rentals in Australia. Will the employer provide any assistance in finding accommodation, for the first few weeks after you arrive and more long term? Would they pay for you to have a relocation agent who can help with those things?
Where is the job based? Where could you live that is within commutable distance? What are the rental prices in those areas? How would you get around? Would you need to buy cars, or could you rely on public transport? What about other travel such as holidays? OH's holiday allowance was much less in Australia, which realistically meant we could use all of his holiday to go somewhere further afield, or hace==ve shorter holidays closer to home. It was also accrued pro-rata which is more common in Australia, so you have to work to earn your holiday. In other words, if you want to take three weeks off to visit the UK, you have to work for nine months to be able to take that leave. The clock then re-sets to zero.
I'm just trying to think of all the different things that either we thought of and researched, or that we didn't think of and which tripped us up a bit so that you don't make the same mistakes.
Australia is a wonderful country, and I do feel lucky that we had the opportunity to live there. However, I would never have gone if I hadn't been sure that we were financially and emotionally able to cope. We've never really lived near family, but I know that others do rely on family for support. Moving is very stressful, and it can be hard to cope without that family support there.