Well Bai Yun is a more experienced panda mom and I've only seen her with the girls not with her male cub Mei Sheng. From everything I have read and what the moderators at the zoo have said - the moms are rougher with the males than the females.
Their play behaviour is actually training for him - she is teaching him skills he would need in the wild to defend himself. She would never hurt him and in fact she will frequently check on him if he takes a tumble to make sure he is okay. She also loves to sleep in the hammock with him cuddled up next to her, really she is a big softie when it comes to her cubs
The male panda in the wild would be isolated whereas the female will have cubs to keep her company during her lifetime. In the wild, the males would compete with each other for a female during breeding season - he would have to know how to fight and defend himself..so this is all to prepare him for his life an adult. In fact several years ago they released a panda back to the wild and he was killed because he wasn't properly prepared to defend himself in the wild. Even though these pandas are in zoos, the plan is eventually to prepare future generations for release to the wild so they have to be strong or they will not stand a chance.
Xi Lan is monitored 24/7 and if they ever thought he was in danger - they would intervene. Lun was a bit rought at times with Lani but she has grown into a beautiful young lady panda and she will no doubt herself be a great mum some day and pass on the skills Lun taught her to her offspring. And from what I've read Mei was just as rough with Tai Shan and he's also turned out to a beautiful panda adult