Firstly I would say you have to talk to your dp, tell him that you are in no way encouraging it and that you want him to be an important part of her life but that your relationship is most important at the moment and you need to be united.
Personally I would not make a big deal about it, he will have to not let Grace show its bothering him too. You can be sneaky with things, like if she asks for something say I need the loo just go and ask Daddy but I wouldn't force the issue. She's probably feeling a bit unsure about what's going on, having just got used to having you apart she's suddenly got you together. She may be jealous of your and dp's relationship, if she's had you each to herself she may be worried he's going to steal you away from her and so she's picked one parent to stick and make sure she keeps so she doesn't loose you both to each other. I hope that doesn't sounds harsh but I know that's not what you are doing but it may what is in her head.
My two fight over holding my hand and dh says I have two hands too, their answer is you can hold the dog then daddy! But when he gets home from work they jump on him and totally ignore me. Perhaps focus on things he would normally do with her that is their thing together or you go out for a few hours and leave them to it. Not necessarily something special just a few hours at home together playing with her toys or baking some cup cakes.
I would say don't try to push her away/towards him or she may feel more insecure about everything. It's going to be hard for him while its like this but he's the adult and she's just a little girl tryin to figure out where everybody belongs again xxx