回複：媽媽四簽終於簽成,能否帶兒子入竟?

There should not be any problem. You may want to write 2 letters:
1) To whom it may concern - that you and your spouse give permission to your mum to bring your son to USA
2) To a US immigration officer that your mum are coming to visit you and should there is any broblem you can be contacted at ...