The direct answer, up front, because it's the only responsible way to open this page: Mr. Grummel is not a parental control. It's a self-directed focus tool that a person uses on their own phone, of their own will, and any block it sets can be lifted by whoever holds the device simply answering a quiz question. A child who wants past a block they didn't choose can do exactly that, in seconds, which makes this the wrong category of tool entirely for restricting a child's access to anything. Search engines send people here looking for exactly that, and the responsible response is a direct answer, not a page that quietly hopes the visitor won't notice the mismatch.
What parents usually try first
Screen time app limits, taking the device away at a set hour, parental control apps with content filtering, router-level restrictions. These vary widely in how well they actually hold up, but they share one property that matters here: they're built and administered by the parent, with a passcode or account the child doesn't control, which is the actual property that makes a restriction a restriction rather than a suggestion. Screen-time apps built for younger children specifically also tend to include age-appropriate content filtering on top of time limits, which a general-purpose focus tool has no concept of at all — it manages when an app opens, not what's inside it, and for a young child those are two very different, and both important, kinds of control. Router-level or DNS-based restrictions at the network level offer another layer some families use, blocking access at the network itself rather than the device, which can catch gaps that a device-level tool alone might miss. Content filtering at the browser or network level, offered by several ISPs and routers directly, adds another layer many families combine with device-level controls for broader coverage.
Why a quiz-gated blocker specifically doesn't fit
The entire mechanism of Mr. Grummel is that the person holding the phone answers three questions to get back in — there's no separate parent account, no hidden passcode, no remote management console. That design is deliberate for its actual purpose, which is self-directed adult and teen focus, but it means there is no meaningful barrier between a child and an app they're not supposed to be using, because the child is also the one being asked the question, and children are frequently better at multiple-choice quizzes than the adults hoping to stop them. It's also worth being clear that this isn't a shortcoming that could be fixed with a setting somewhere — it's a structural property of how the product works, built for a self-directed adult or teen user making their own choice, and no configuration turns it into a tool a parent can lock a young child out of on their own. That's worth repeating in different words because it's the single most important thing on this page: nothing about how this product is configured changes who's actually answering the quiz question when the phone is in a child's hands. No amount of marketing copy changes that underlying mechanical fact, and this page won't pretend otherwise for the sake of a better-sounding pitch.
The honest fit
There genuinely isn't one here for the core parental-restriction use case, and this page exists specifically to say so plainly rather than let a search result imply otherwise. If a family finds this page while searching for a way to manage a young child's phone use, the honest answer is to leave and use a tool actually built for that job. Rather than stretch the description to sound like it fits, the more useful thing this page can do is say clearly that it doesn't, and point toward what actually does — which is the point of the section directly below. A search result that implies otherwise, even accidentally, does families a disservice by suggesting a false sense of security where none is warranted. Better to lose a visitor here than to let them walk away with false confidence in a tool that can't do the job they came looking for.
What to use instead
Apple's Screen Time, set up through Family Sharing, lets a parent set app limits, downtime, and content restrictions locked behind a separate passcode the child doesn't have. Google's Family Link does the same job on Android, with app approval, time limits, and location features designed around a parent-managed account. Both are free, built by the platform holders, and are the right starting point for anyone who came to this page looking for that instead. Both are free, both are maintained by the company that makes the operating system itself, and both are specifically designed around the asymmetry that matters here: one account holds the passcode, and it isn't the child's. Setting either of them up takes a few minutes through the respective platform's Family settings, and both are worth doing before relying on anything else.