Yes! And grown-ups can also sit and even stand on these blocks. Just make sure you even out the weight by covering the blocks with a flat surface. Certified average weight hold is 59 lb. / 27kg for an XXL block and 53 lb. / 24kg for the XL version.
The only difference is in size.
XL is 60% from XXL block size.
XXL are 7.87x3.94x3.94in (20x10x10cm).
XL are 5.11x2.56x2.56in (13x6.5x6.5cm).