Mezuman and kashering

Question

1. according to the Shulhan Arukh Orah Hayyim 189:10 you can include a child in a mezuman as long as he knows who he is blessing. This is certainly the case with a 5 year old child. How come we aren't accustomed to do this? How old should a katan be before including him in a mezuman. 2. Are there any differences with regard to Sephardim and Ashkenazim when it comes to kashering items between milchig and fleishig (or the reverse). Someone told me that only Sephardim are allowed to kasher something from milchig to fleishig. Is this true?