To observe present and past is child's play (6)
I believe the answer is:
seesaw
I believe this clue is a double definition.
'to observe present and past' is the first definition.
(see present tense and saw past tense)
'is child's play' is the second definition.
(Other definitions for seesaw that I've seen before include "Children's up-and-down play structure" , "Pivoted amusement" , "Playground device" , "Teeter-totter" , "Playground balancing board" .)