Ok I found out what was missing. So my approach was right (and it is similar to yours but more generic), thing is, in my BaseActivity:

abstract class BaseActivity<ViewBinding : ViewDataBinding, ViewModel : BaseViewModel> : AppCompatActivity(), HasAndroidInjector {

    @Inject
    lateinit var dispatchingAndroidInjector: DispatchingAndroidInjector<Any>
    @Inject
    lateinit var viewModelFactory: ViewModelFactory
    protected val viewModel: ViewModel by lazy { ViewModelProvider(this, viewModelFactory).get(getViewModelClass()) }
    protected val binding: ViewBinding by lazy { DataBindingUtil.setContentView<ViewBinding>(this, getLayoutResId()) }
...

I’m providing a Factory just for the ViewModel received by parameter from the ChildActivity. To be able to use N ViewModels, it’s ChildActivity responsibility to provide them just like the Base does.

I also have a generic ViewModelFactory:

class ViewModelFactory @Inject constructor(
    private val viewModelMap: MutableMap<Class<out ViewModel>, ViewModelAssistedFactory<out ViewModel>>,
    owner: SavedStateRegistryOwner,
    defaultArgs: Bundle?
) : AbstractSavedStateViewModelFactory(owner, defaultArgs) {

    @Throws(IllegalStateException::class)
    @Suppress("UNCHECKED_CAST")
    override fun <T : ViewModel?> create(key: String, modelClass: Class<T>, handle: SavedStateHandle): T {
        return viewModelMap[modelClass]?.create(handle) as? T ?: throw IllegalStateException("Unknown ViewModel class")
    }
}

A generic ViewModelAssitedFactory:

interface ViewModelAssistedFactory<T : ViewModel> {
    fun create(stateHandle: SavedStateHandle): T
}

And every ViewModel just have to add this lines:

@AssistedInject.Factory
interface Factory : ViewModelAssistedFactory<MyViewModel>

The rest will be handled by Dagger :)