So if she's not his, are you hoping he won't see her anymore? What if he doesn't agree to that?

Toys=Love to your SD, we know this isn't true but to her it is. She is announcing that she is more loved. Why? Because her parents aren't together- it is all a competition. Your DD has mommy and daddy all the time. SD doesn't. I am not saying poor pitiful SD the COD, I am saying for a young child, this is common and makes sense. 

Try not to let those comments get any reaction- Just nod, and say "Cool story". Then move on. 

What are the plans if SD isn't his? 

I disagree with above and would start teaching her there is more to value in life than just stuff.

testing or was it more something you wanted to do? Did you and DH discuss what actions would be taken if it turns out he is not the biological father? 

I'm sorry but I am not super familiar with your story, was DH married to BM when SD was born? It varies state to state on what can be done legally if it turns out he is not biologically her father. Some states you have 60 days, some states you have 2 years, and other states like mine (VA) at any point in time you can do something about being proven by genetic testing that the man is not biologically the father of the child. In some states in matters if it was assumed paternity by way of marriage and other states if you acknowledge paternity then you can't get out of it. Really is a state by state matter. 

My bf's rights were disestablished to BM's older child during their divorce and custody battle. When bf filed to divorce BM, BM put in writing the child was not his after passing her off as his for 3 years, she got a court ordered genetic test, then took his rights away. Bf did not have a choice in the matter in terms of his rights, where your DH does. Once bf's rights were taken away, he decided to not pursue a relationship with the child even though BM tried to emotionally blackmail him into one. She wanted 100% legal right to the child, but wanted bf to still play dad and that was not going to happen. It wouldn't be fair to either the child and especially not my bf.

My advice to you is if you have not already discussed what happens if it turns out the child is not his, you should before the results come back. My bf had a hard time dealing with the loss of the child who was 4 at the time his rights were officially taken away (she was 3 when he found out she was not his) and your DH's child is 6, I can't even imagine if the child was any older, how bf would handle it all. Bf is doing okay on it now, it has been 8 months, but there are times that it still makes him sad, obviously. 

Going into our relationship, I knew the child was not bf's and I accepted that and the child. I never pressured him to give up his rights to her, which is what BM wanted, just to make the divorce go by faster. There were times I wish he would just give her up because it was so stressful and hard, plus I wanted him to be divorced, but I did not because I did not want to bf to resent me or make him make a choice he really did not want to make. Even though it looked like it was going to end up that way anyway, which it did. The divorce took 2+ years because of this paternity issue and while yes I wish it didn't take this long, since bf did not gain anything more at the end then he would of if he just agreed to give up his rights from the beginning, I would not take back supporting my bf fighting for the child. Yes, he did lose her, but he doesn't wonder "what if" and he can't say he gave up, he tried and he lost, but he lost doing what he thought was right and what he wanted to do without my influence.

I tell you all of this because while you are your DH's partner and spouse, your DH has thought for 6 years this child was his, and ultimately it should at the end of the day be his choice on what to do with the information. Of course you can put in your thoughts on the matter into the conversation, but I would not give him any ultimatums or anything like that. For the health of your relationship and even your DH, it really needs to be his decision. There are times bf says I wish I just gave BM what she wanted from the beginning instead of wasting all this time, money, and putting you (aka me, futurestepmomnowstepgf) through all this since I ended up with the same result anyway. But I 100% am happy he tried because of that we can move forward without it tainting our relationship.

On the subject of the toys issue with SD, I agree with the others. It is because SD does not get to spend all her time with both her mom and dad, the more toys in her mind makes her feel loved and special. Of course, do not give her more toys or anything like that just because she is a COD, but there is no malicious intent behind it. Kids compete and especially a child who feels maybe not as special because to them looks like their little sibling has it all, just need reassurance from time to time that they are also special.

Take or leave my advice, just know I am not trying to downplay your emotions, thoughts, or feelings on the matter, but I am trying to share my experience from a person who has been there. Yes, our situations are different, but I have been through the rollercoaster of emotions on both my side and my SO’s side on what to do when a child believed to be theirs is not. I just hate for your relationship to be negatively affected by the strong emotions you are feeling. If I didn’t have this site and the friend I vented to during all this, I would have been lost and torn on what to do and whether it is okay to feel how I was feeling.