You should seize on this as a perfect time to have one-on-one learning time with your 7 year old.

Don’t use an app. Don’t show a movie.

Teach the kid yourself. This can be meaningful as a parent-child interaction.

I agree with @elbanditoroso. Also, take her age into consideration and don’t make too big of a deal out of it.

I don’t know if the follwing helps.
https://www.lifewire.com/how-to-turn-off-auto-correct-on-the-ipad-4103807

Auto correct only corrects spelling, not punctuation. But even if it did correct punctuation, the child wouldn’t learn proper punctuation for herself.

Even if parents interact with their children to teach them things, letting them use a software that shows the correct way is additional help.

I just posted an OP regarding the topic.
https://www.fluther.com/218407/which-punctuation-correcting-programs-are-betterbest/

Age 7 is very young for mastery of punctuation, capitalization, etc. I homeschooled my kids for 10 years. Every child learns at their own pace.

You might show her how punctuation helps. Try writing a passage without any caps or punctuation. Then read it out loud with her. Ask her how you can know when to take a breath. Find a way to turn it into a game and make it fun. If it’s not fun she will hate it, and you might as well forget it. Also, be sure to introduce only one concept at a time. Go slow, make study times short, and have lots of rewards. Periods today, capitalize a few days later when she gets the idea of periods. Then move on to commas, etc.

Also, is she writing on an iPad? That right there could be your problem. Give her a pad of paper and a pencil, and let her write that way.

When my son was learning to read I once had him read a book to me. He got to the end of a sentence and he fell silent and started tapping his foot.
After a moment I said, “What are you doing?”
He said, “See that? That’s a period. That means ‘stop.’” :D
BTW, he was about 7 when he finally learned to read. They weren’t teaching phonics at that time, so when he got to the end of 1st grade and he still couldn’t read, I took matters into my own hands. Had him reading inside of 2 weeks.

Hey, that’s a thought. Use your body language to show commas (pauses) and periods (full stop.) Also, her name is very important so that’s why we capitalize the first letter. ALL names are very important.
This needs to be a hands-on lesson, though.

One of these example might help your child see it when it comes to the comma:
“Let’s eat gramma.” versus “Let’s eat, gramma” good example
https://mashable.com/2012/09/24/misuses-punctuation/
https://digitalsynopsis.com/tools/punctuation-marks-importance-rules-usage/