I always just assumed Arvis gave the weapons to the kids because he thought he would be a good emperor, so he didn't fear any rebellions. He thought the people would be happy. It could even benefit him, Faval almost ends up fighting for him with Ichival. Seliph is an exception because he is the rightful emperor, so to Arvis he is the only one with a legit reason to rebel so that's why doesn't give the Tyrfing back right away. Maybe he found out Shanan hangs out with Seliph and that's why he kept Balmung from him.
The thing about Arvis is, he's willing to do evil things to accomplish his goals, but not any more then neccesary. So betraying and murdering a bunch of people is fine but stealing their weapons just cus he can is something he wouldn't do.
Also, when Ethlyn goes to the desert in chapter 5 she doesn't have her weapons with her. She left them in Leonster so that's why Leif gets them.