All the calling of the ice-cream guy would tend to confirm is that the complainant bought her siblings ice-cream. That doesn’t prove that she bought them ice-cream in order to seduce Tom.
She would say she bought them ice-cream because she is a nice sister.
I can’t remember if the complainant was questioned by Atticus about this at the trial, but in a real trial she would have to be before the matter could be taken further evidentially.
There are then two possibilities, in reality. She might deny having given the kids the money to buy ice-cream, or she might admit it, saying she was a nice sister, and that it had nothing to do with the sex.
If she agreed she did it, then there is no need to call the ice-cream guy. There is a rule against multiplying issues, sometimes called the finality rule. If there is an issue that only goes to a witness’s credit (as opposed to a matter directly related to proof in the case) then there are limits to the extent to which it can be litigated beyond the ipse dixit of the witness whose credit is challenged.
Suppose a rape complainant says she was wearing a red dress and the defence wants to suggest it was blue to challenge her believability on other more important issues. They ask her. And they are stuck with the answer they get. They can’t derail the trial into a trial about the complainant’s colour vision accuity, her wardrobe contents, her recent purchases of clothes, her taste in general, merely to make a minor point.
On the other hand if the complainant says the suspect was wearing red when there is other evidence the offender wore blue, that can be litigated because it goes to the issue of identity, which is essential to proof.
The difference between peripheral matters of credit and matters central to proof can be difficult to discern, and there is a degree of leeway sometimes offered in such cases, but buying the kids icecream may well only go peripherally to the issue of consent.
Now if she denied saving up to buy the kid’s ice-cream altogether, the story might be different (there are exceptions to the finality rule). But the problem is not solved for the defence by calling the ice-cream guy. His evidence would not be sufficient to prove where the money came from. The mere fact that the kids turned up with money does not establish who gave it to them. And even if the kids told the ice-cream guy where it came from, it would be hearsay for the ice-cream guy to repeat that. Unless the kids who were given the money were called, this is likely going nowhere.
It would be a brave counsel who called a girl’s younger siblings to give evidence that would essentially call her a tramp (in the environment of this trial). It has such obvious possibilities of back-firing even if the kids could be got to court that many counsel would not risk it.