Stepchildren visiting US

I married an Australian with 3 step children a few years a go. We are moving back to the US and have received a Visa for my partner. However during the process the ex would not agree to visitation rights. Now after the visa has been granted they have agreed to visitation.

Do the 3 step children need to get a visa to visit for more then 90 days? If so which one? Can I apply for them when we are in the US?

Legally, they will need a visa to stay for more than 90 days. Probably the easiest one to get would be a B1 or B2 visa which they would have to apply for in Australia. These are tourist visas which don't cost a lot of money and usually last for six months. Basically, they'd have to prove to the embassy in Australia that they have no intentions of trying to stay in America permanently or obtaining employment, and that they can financially support themselves while there without resorting to working in America. A good letter from you and your partner, stating that you will be supporting them while they are in the USA would probably suffice. The alternative would be for them to fly to America and just stay longer than 90 days. If they went back home after 120 days or 150 days, since they're leaving the country anyway, no one will care. US Immigration is probably the most inept department on the face of this planet and they usually go to a great deal of trouble to persistently bark up the wrong trees, but even they won't care about someone who stays a bit longer than expected or stated when that someone is flying home anyway.