Hi Ricardo,
I am not sure which country payroll you are using, but it sounds like you might need to create a couple of functions to gather the data you need (get children information from 0021, get plan information from 377 and read the children's data from 0021 for the age calculation referred to earlier in this chain). I feel that all of this enhancement work would be better placed into supporting a process that keeps the entry from happening if the conditions you mention are not met.
I would look at setting up the checks that you are trying to apply in the entry process for the wage type. In a PAI user exit for IT0014 and/or IT0015 execute the checks for the plan and then evaluate the records for children to determine A) if there are any children and B) does any one of the children meet the age criteria. If the criteria are not met (a 377 AND a child under the age of 6 for the periods in question) then you generate an error message. If there are children then determine the latest date of eligibility and do not let the IT be created after that date.
Then - if the wage type exists you can process it as required, it can only be available in payroll if the system allows the creation of the record.
Cheers,