a) they don't know how to use commas
or b) they aren't checking their work
If you already know the the answer is b, then you can read a little more about checking work here. If you're not sure, because due to remote learning children may have forgotten or have simply fallen into bad habits, you can use this four-slide PowerPoint to check.
1) Ask the child to read the short passage on slide 1 and point out where the commas go.
2) Check the answers on slide 2.
If they have identified the comma placement correctly, clearly the issue is the they need to check their work more carefully. Again, you can find more information here about how you can make that happen (because let's face it, they'll need a bit of coaxing to do it properly) here.
If they are struggling to get the answers right, you can use slides 3 and 4, which show the verbs and main clause structures respectively, to help them understand how commas are used to separate clauses in a sentence.
You can download the PowerPoint absolutely free here.
Have fun!