& Terry’s response basically sums up my thoughts on this -
for someone topping up your card to be a real problem, they’d also need your card so that they could spend the money or they’d have to know that topping up your Monzo card would cause issues for you & really not like you because there’s no financial incentive for them to do so.
The possibility of either happening seems so remote that the extra friction just doesn’t seem worth it.
If you’re an Android user & you let your child play with the app / regularly give it to someone else while it’s unlocked, you can use an app like Norton app lock to protect it.
And remember that you won’t be liable for any fraudulent transactions, if your card is stolen, see Terms & Conditions