I assume from your use of 'my child' that your husband is your daughter's step-dad? I think its sad that families have to worry about stuff like this - you wouldnt have married him if you didnt trust him would you?
I am in a similar position living in a step family. I would let your daughter decide who baths her - she will let you know who she feels comfortable being bathed by - in a couple of years time she will want to bath herself anyway. Let her lead your decision.