How much income do I need to have?

[Terpe]

..Is the figure £18,600 Gross or Net pay?

It's gross annual income.

..If Im unable to earn £18,600 from one position, is it possible for me to obtain a 2nd position, part time work on the weekend and make it up to £18,600

Yes, provided both positions meet the requirements of the category you are applying under.

..Plus get my mum to put in a sponsorship too to make the application more stronger.

Third party sponsorship is NOT allowed.
Of course you may live in your Mum's house etc, but she is not allowed to provide any financial support.

..After my wife comes here with our two british kids my wife will work weekends and i can then look after the kids on the weekends.

Nothing wrong with that, but it has no influence or impact on the Spouse visa application.
Your wife's income can be counted when you apply for FLR after 30 months.

..What do you think ? is it possible?

Yep. It's possible.

Aiden, it's really important that you do some self-research.
No-one here is a qualified immigration advisor, we can only point you in the direction and help you with very specific questions on the understanding of UKBA rules.