Schengen Visa for Thai step daughter

[bigyin]

If I want to obtain Schengen visas from the UK for my Thai wife I think I have it straight that for most countries I will need to get the translation of the marriage certificate certified/legalised at either MOFA in Bangkok or the Thai Embassy in London.   Am I correct?  However, what do I need for my step daughter and does me not being the biological father cause a problem?  My thinking was the  translations of her birth certificate, change of name from the father's name to my wife's and the letter we have from the amphor in Thailand confirming my wife has sole custody and freedom to take her daughter out of Thailand.  Has anyone done this or have any ideas?  As a side issue has anyone sent their step child on a school trip to another EU country and if so how does that work?