I’ll throw in a suggestion too. Maybe kind of a hybrid of the other options. You’ll have to use a custom form for this as it will have to update the sheet in real time. What I would maybe do is still write one row per task, but you could probably have a checklist of children. The checklist would be attached to the ParentChild sheet and the check box would be attached to a user specific column in the ParentChild sheet. You could check off each child the task pertains to, then pull that into the sheet you are using for your form through a relation and a joined list. This way you could have a comma delimited list of children in your DataLog that you can then split and use for a relation back to the ParentChild sheet.
That’s a rough example and has some limitations. I would check out this thread and maybe take a look at @Roldy’s example where he saves the selections.