[Q] How to mount ext sdcard as read-only in 4.1.2

Hi,

I have a Samsung Galaxy S3 verizon model. Its rooted and running CleanROM 6.0R2 ROM android 4.1.2.

I am trying to accomplish below

1. Mount external sdcard in read-only mode. This is to avoid files on the extsdcard being accidentally deleted.
I have music and some old photos in there. Sometimes my son plays with the phone. I don't want him
to delete all the photos/music on the external sdcard.

I have tried following, but none of then resolved the issue

1. Partition external sd card as ext3 and set all files/dir to read-only permission from ubuntu box.
This didn't work because, phone doesn't recognize the sdcard and partitions on it.
2. Partition extsdcard as vfat and set permission for the files/dir to read-only using root-explorer.
I cannot set permissions for vfat partition.
3. Partition extsdcard as vfat and Mount extsdcard as read-only using "root-explorer".
Doesn't work. Files can be deleted from other apps like Gallery.
4. Partition extsdcard as vfat and mount extsdcard as read-only using terminal and busybox.
This works. But files are owned by root. For group and other, permission is None. Hence
other apps are not able to view the files.

From reading many posts, I conclude, partition has to be vfat for apps like Gallery/video player etc to view it.
Only option to achieve read-only on a vfat partition is to mount it read-only. Based on this, any idea how I can achieve my objective?
Also, once set, what happens is the phone is rebooted? Anyway to repeat the sets at boot time?

Thanks