Chinese partner British national son

[oscar35]

I am a male British national from birth. I am also married to a UK national from birth, but we are affectively separated as we now live apart. I have a female Chinese national from birth partner, we have been together for circa 7 years, but due to me being self employed running a company in the UK for the earlier part of our relationship we were apart more than together. One the last 4 years we have been together more me visiting China and she visiting the UK on a 6 month travel visa. In October 2017 we had a son together, which was born in China and through me being a British national he obtained a UK passport and was a British National from birth (he has a permit to travel currently from China not recognised or needed in the UK).
My partner visited us for our sons first birthday on a new travel visa and our son has enrolled at a nursery two days a week. We both want our son to stay at the nursery and have a UK education.
My partner cannot use her travel to continue visiting as it could possibly breach the continuous use rule. What visa could she apply for in view that I am still married.