There are many ways to accomplish that, one way would be
submodel = nn.Sequential(
    model.features,
    model.conv6,
    model.relu,
    model.conv7,
    model.relu
)

To get the output of relu(conv7). Children will return registered submodules, which means as they are added in __init__. Since relu is applied multiple times in forward, you can’t just take children.
Another way would be to just return the intermediate values along with final value in the forward.