How can I let a child use iPhone Lock Screen without risk of disabling it?

Just don't.

Aside from managing to lock you out, it's highly likely that the phone will be physically damaged when it's dropped, thrown, spit up on, or otherwise abused.

You could put the phone in Guided Access mode which will let you restrict her access to one app you've chosen.

Use Guided Access with iPhone, iPad, and iPod touch - Apple Support

But first, as Niel says, back up your phone.

Back the iPhone up before giving it to her. Anything you've stored on it can be restored from that, whether the unit gets disabled or data deleted in other ways.

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You should be concerned that random pushes on your iPhone screen could cause you to be locked out of your iPhone. Niel's advice is good to suggest you make sure your phone is fully backed up before you choose to let your granddaughter play with your phone. But the hassle it could cause is truly not worth the fun your granddaughter might get from playing with your phone. No matter how much I loved a child, I would NEVER EVER let them play with my phone. They aren't toys and certain actions can cause painful to recover from situations.

Your choice of course.