I think that ultimately you do need to let them play with the toothbrush a bit. If they let you brush their teeth afterwards, then great. But if you can get that brush into their mouth without protest, then you've achieved something. It will improve in time.

We haven't yet started using toothpaste for Sam (probably should, but I still think he'd just eat it!), but he comes with us to brush our teeth when we do. Most of the time he just sucks the water of his toothbrush and asks for more. But if we keep wetting his brush while we do ours, when we are finished he will let us brush his teeth for him. But if we just take him to the bathroom and ask him to open his mouth so we can brush his teeth - not happening! Letting him have his go keeps him happy enough to let us follow up later. Doesn't always happen, but happens enough to keep us happy.

BW