Are these suitable for children or adults?

Sure, I think there's no age limits.

Would these work for slip cover to hold fabric tight under a dining chair?

I would expect them to work fine, at least for a while. There are small plastic "teeth" that grip the fabric. If the fabric is a loose weave there may be some penetration of the weave and possible damage, but if it is under the chair seat will it matter. The elastic is adjustable from about 7.5 inches to about 6 inches including clips before stretching. It will stretch about 1 more inch. The clips hold the hem of my jeans down over my boots at over 60 mph so I would guess they will hold a slip cover.

Could these make a suitable hat clip?

I use them to keep my pants legs down while I'm riding a Harley I'm not sure what a hat clip is.

Can this allow flexibility for cosplay with black tactical pants i want to look tapered and snug ?

As long as the clip can sink it's teeth into a thick hemline or bulk of fabric then yes the straps are elastic and will provide good flex. The key is in what the clips can grip in to so it doesn't slide off edge under tension.

ASIN	B07GWJ4GF5
Best Sellers Rank	#219,949 in Clothing, Shoes & Jewelry ( See Top 100 in Clothing, Shoes & Jewelry ) #13,962 in Women's Accessories
Brand Name	Boncas
Color	Biker 4.7" -9"
Customer Reviews	4.0 4.0 out of 5 stars (2,859)
Date First Available	September 10, 2018
Item Package Dimensions L x W x H	4.65 x 3.15 x 1.06 inches
Item Weight	0.2 Pounds
Manufacturer	Boncas
Material	Elasticized-Fabric
Outer Material	Elasticized-Fabric
Package Weight	0.09 Kilograms
Part Number	BC1053500US219KF
Size	Biker 4.7" -9"
Suggested Users	womens

Boncas Improved Adjustable Elastic Boot Clips Boot Straps Pant Clips Stirrups Leg Straps Keeping Your Pants Tucked

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