Hi peeps

Im a newbie but do have some experience rooting a tablet.  I have a kindle hdx 8.9 and the os sucked, so I went to xda and read through alot, and asked alot of questions and eventually rooted my kindle and it is now running  as an android... and I am so happy witth it.... its mainly so I could watch kodi on it with a great screen spec... now all this is leading up to my question.

I bought 2 Samsung galaxy tab a 7" screens for my 2 autistic grandsons...and 128gb minisd cards each to store apps on it as it only has 8gb.  My daughter emailed me and asked if theres anyway to remove alot of samsung programs, because even tho shes moving the apps to the sd card. she keeps getting messages saying low on memory ..SO what can I do to these tablets to help with increasing the storage size so these messages dont pop up... as autistic children dont understand that they need to let mom sort it out for them, 

I've read about adoptive storage and get conflicting videos on you tube as to what to do... and the best way to do this.  I dont know what version is running but can find this out and anything else thats needed... my daughter is coming down this weekend so I can hopefully work on the tablets then...but they were bought dec 2016 for christmas, brand new.

So can I get rid of alot of the useless programs on the samsung as the boys are 4 and 5 yrs old and love gaming, watching you tube videos on car racing, and mom wants to be able to put films on the sdcard for when she needs a break lol...

I'm more than happy to pay/donate to get the right information... and I know that by doing anything to the tablets is my responsibility and nobody elses.. I am great at following steps or even bullet points in order ...

Lastly, if theres a ROM that can be flashed that frees up space instead of deleting apps, then I would love to learn about that as well...

Ive just read about deodexing a rom.... what is the reason for this ?

Many thanks, and sorry for the waffle...