Those examples are the "cost of doing business"...just like any restaurant.How sad that you have already decided to stiff the crew.
Yes...kids should be well-behaved and as tidy as possible for dining, but as most folks know, that doesn't happen all the time. Nonetheless, it's also something already accounted for in 'dining clean-up". I've seen enough adult sloppy/messy eaters to know it's not exclusive to kids either.
We are traveling with a 20-year-old young adult and 4-year old grandsons next time...and there's no way we can begin to reason justification for the 4-year-old gratuity that would exceed $100+ - rest assured...we will contest that amount.
In contrast...we have no issue paying the "normal" rates, including for the 20-year old. We've been known to even pay extra when warranted by the service levels. So this becomes an issue of principal, not actual dollars with us.
The 4-year-olds will still be eating, bathing, sleeping, etc. Why shouldn't they be charged tips like everyone else.
I sail solo - pretty much pay double rates for a cabin. Solo cruisers don't get a break because they are not eating and drinking (included beverages) for two.
It's just something you accept and budget for; or don't cruise.