Is there a chance the trainer can come to your house to watch the interaction between your child and dog? Somehow the dog is not respecting her as a leader. And perhaps there was some (bad) interaction between the two that you are unaware of that left your dog not trusting her?
I definitely wouldn't leave the two unattended together. 6 years old, even without this problem, is a little young to be left unattended with a dog. Especially a big dog. I would be watching the two together like a hawk. Watching for signs on both of their parts. Children don't understand the body language of a dog. Some grown ups don't. They may pester too much or get in their face when the dog doesn't want it. They don't know when to back off. They're just children. I would be watching their interaction to see if I could pick up on what's happening. Who's at fault. This is where the trainer might come in handy.
Our Annie is 10 months old and we have five grandchildren. Three who live close by and are here often. The youngest is 8 years old . Annie has been wonderful with them. But I constantly watch! Both the children and Annie. She was corrected from the get-go about jumping on them or
anyone for that matter. And once in awhile the children are corrected too if I feel they did something wrong or did it the wrong way. I try to get them to give commands to her. Even just to sit. To help make them a leader over the dog. She needs to respect them and not think of them as a plaything. Even though she loves them and wants to play constantly!

It's very important to me, as I'm sure it is to you, that my dog gets along with children. They are part of my life that are not going away. So the dog needs to fit in. And that's my job.
8 months old seems very young to me to be 'aggressive'. Like
@JanS said I would ask the trainer if there was anything that may have happened while he was away. I've never sent my dogs away for training so I don't know what that's like. But I would think that the training was involve socialization and maybe even children. But maybe not. That would have been nice though!

That's an important part of training to me.
I don't know, I'm trying to put myself in your position as to what I would do. And the biggest thing that comes to my mind is being the third party whenever your child and dog are together. I would be making sure your dog minds and is calm and settled before any interaction.
And remember the rule NILIF (Nothing in life is free.) Annnie doesn't get anything unless she gives me something. Potty training was always, give me a potty and then you get a treat!:

You want a bone to chew? Okay lets go through our Sit, Stay, Down, heel, whatever. She has to do
something. I don't just hand it to her.
Is your dog allowed on the couch? Being higher than your child as she walks by might be an issue. It gives them superiority. I'd be watching that too.
Check out your dog's health first to eliminate that and go from there. Please keep us updated on your progress.