To show subscription widgets in your bundle widgets, simply make sure that:

1.) You have the Seal Subscriptions app installed (https://apps.shopify.com/seal-subscriptions)

2.) That you turned on the integration in Bundler > Settings > Subscriptions.

3.) Make sure that you added all products from the bundle into a subscription rule in the Seal Subscriptions app. 

If these 3 conditions are fulfilled, then the subscription widget will show up automatically in your bundle widgets.