How to become more authoritative while babysitting?

Hi,
I babysat two boys tonight and I slightly messed up. I got them to bed on time, wasn't on my phone, made sure they didn't watch T.V but one of the kids wasn't allowed to watch TV or play with any electronics. The sucker I am let the kid play on an ipad… I know i shouldn't have and I feel so guilty. The problem is that I have trouble saying no. It's absolutely awful and I'm not assertive enough because I'm scared that they won't like me. I know the parents rules are the most important thing after their safety and that's what's making me feel so bad. I really don't want one of the kids to say something because I let him watch a few videos and then I get fired. I'm so scared of getting fired because of that one rule. How do I be more authoritative and not let kids walk all over me? Thanks!

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