people that will laugh at anything
Ridibund
From Wiktionary
Inclined to and easily brought to laughter; happy.
This word is rarely used so I recommend that you stick with more natural phrases like quick to laughter.
Abderian: Given to laughter; inclined to foolish or incessant merriment.
Based on Abdera
Cachinnating is to "to laugh aloud, laugh immoderately"
You might say they would laugh to see a pudding crawl:-
Someone who would laugh to see a pudding crawl is easily amused and will laugh at anything.
(See also here.)