That your spouse and child join you in UK and live with you as dependents is your own wish and not compulsory / mandatory requirement

. So yes, your family member(s) can apply for visitor visa (family) if you / they wish so.
If you are applying for one family member as a dependent and the other as a visitor - specially a child and the mother, you might have to convince the ECO as to why you are doing so (a well drafted cover letter perhaps). Not suggesting that there will be objection(s) for sure, but there are some obvious questions that come to mind and it helps thinking out of the box and preparing oneself beforehand.
Also, please bear in mind that the family member on visitor visa will not be able to switch to dependent status from within UK. That family member will have to return to home country and apply for dependent visa.
Another point to bear in mind is that for the visitor visa application also, as a sponsor you will have to provide evidence that you will be able to accommodate and maintain the visitor without recourse to public resources. Usually, one has to submit 3-6 months' payslips and bank statements, and provide proof of adequate accommodation (e.g. - rental agreement) amongst other documentation.
You will have to figure out how to make two different visa applications (dependent and visitor) at the same time / within few days of each other and yet convince the ECO that you have adequate existing financial resources to maintain a dependent and also a visitor.
If you can address these concerns (as mentioned) skillfully, based on my personal experience I can say that there shouldn't be an issue at all.
regards