Mum to accompany tier 2 dependant

Hello,

Hope anybody can help. I wrote here sometime ago about my plan to bring my 13yo son to join me here in the UK. What I was planning initially is to go home and come back with him. However I found that very expensive. So now I am planning to let my 63 yo mum (who has been my sons guardian for more than 5yrs now-whilst I am working abroad) to accompany him to come here. She has not been to the UK so this is a good opportunity for her to tour the country.
My worry is that since she will be applying a different type of visa (general visitor visa) compared to my son(tier 2 dependent), is there any chance that she wll be denied? I fear that this is greatly affect my sons application as I will put her name down as the accompanying person. Do I have to prepare anything? I know about the general requirements (financial) that she and I need to meet but Is there anything specific?

Thanks!