Regularising Baby's Stay in UK

I am currently working in the UK under the Tier 2 general visa. I went to have my baby in USA earlier this year and obtained US passport for the baby immediately. The US passport (without visa) was used to bring the baby back to UK with me. This is based on the fact that US citizens can enter into UK without visa for 180days. My Tier 2 general visa is still valid till 2019. How can I go about regularizing my baby's stay in UK.

Please help. Thanks

You should have applied for the Tier 2 dependent visa from the USA.

It is usually not possible to switch from 'visitor visa' status to any other visa category within the UK.

Your child is on a visitor visa, granted on entry to the UK.

What can I do now?

Would it have been easy to get the visa in US despite the fact that I travelled to US on a B1/B2 visa?

You can apply in the UK and hope for the best.