subscription

Without knowing the specific subscriptions you are needing to cancel will limit the steps you would have to take. Generally, if you have an active subscription, you can cancel it without paying more for it (unless there's a contract that states otherwise).

If the subscription(s) you need to cancel are connected to your Apple ID, here are the steps:

Go to Settings and click on your name at at the top to get to your Apple ID. You may be asked to put in your Apple ID password.

The fourth option down is Subscriptions, click on that. Any active subscriptions will be listed here and any inactive subscriptions will be in its own list. click on the active subscription and then click "Cancel Subscription" at at the bottom and confirm to stop any further charges.

Here are a couple of options. You can set it up so that she would need your permission to make a purchase:

Approve what kids buy with Ask to Buy - Apple Support

This is specific to Roblox on making a limit as to how much can be spent :

https://en.help.roblox.com/hc/en-us/articles/4409125091348-Monthly-Spend-Restrictions

If you are sharing your subscription(s) through family sharing, you starting or stopping a subscription will control it for everyone. If your daughter has her own Apple ID that she signs up for her own subscriptions, then she would have to cancel it herself.

Again, this is only assuming that the subscriptions are something associated with an Apple ID. There are a multitude of services and programs that have subscriptions that are independent of anything Apple and would need to be activated or cancelled in a separate manner.

To answer your question about another person to consent? That would only be needed if the person wanting a subscription is a minor and the other person would have to be a parent or guardian.

Without specifics on the programs that are being in question, this limits my answers.