the reason is literally simple and already stated 

goku himself says that on a alive body 
the energy drain from ssj3 is extremely taxing thus making the form pointless regardless of amp in strength and power 

vegeta told trunks that the reason he didn’t use grade 2 ssj was because it makes you physically slower thus defeating the point of even having the form in the first place 
( granted he had already perfected it with “ super vegeta “ )

i say that is to point out that vegeta is a tactician. he will not do something unless he knows it will wrk in his favor.

my speculation for this moment however 
is that as children 
the power scale wouldn’t be that taxing because there isn’t enough to put out anyways ( smaller bodies = less energy can be used ) thus giving room for ssj3 to be usable without heavy drain 
their still powerful by normal standards 
but they definitely are not outputting the same amount of energy they would as adults considering their adult bodies are used to larger outputs