One of the nice things about Aikido is that once your partner has applied the technique to you, you then get to apply the technique to your patner. It is a cyclical relationship. If you have told these younger folks to lighten up and they do not you have a few options. Your first step would be to bring it to the attention of the instructor. If the instructor doesn't make it clear to all students that they need to be careful with their practice then I would find a new dojo. If the instructor does tell the students and they still choose to be rough then when it is your turn you reciprocate.
IF they dont injure you first. I think that the custom of letting the senior student be nage first is wise that way, so they have a chance to set a safe and appropriate training tone with less chance of injuring their uke. And uke gets to see the technique a couple more times, as long as sempai is behaving correctly.....