I don't it is unreasonable to have a parent adjust their ways in order for you to maintain care of a group of 5 young children. Skinned knees at home is one thing but skinned knees with 5 kids running around means you have to direct attention to the skinned knees and not the group. If you can easily prevent the skinned knees then it is reasonable to do so. It's not like it is extremely hot, you are transitioning into a new season...kids going from padded snowpants straight to shorts will inevitably skin their knees. That layer of pants helps wonders until the kids learn their footing and figure out how to manage bare pavement.

Obviously if they truly don't have any pants (even some that fit but are small) then their child would have to have skinned knees. Perhaps the mom would see your point when the child is bloody at pick up each day. But it is such a simple request for the mom to accommodate.

Hopefully you just caught her at a bad moment and she comes around.