Hello and Thank You for consulting Just Answer. I would be more than happy to assist you today as one of the Parenting Experts on this site. To tell you the truth, my son had this same problem around the same age. He is 12 now and perfectly fine so your granddaughter will whether this storm eventually.
First of all, the pre school should attempt to be on the same page to help her go to the bathroom. Maybe the pre school can use a sticker chart when she has a BM. Then after so many stickers, about 5 to start, she can take the sticker chart home and be rewarded by her parents. The school does not have to be responsible for the rewards...just the stickers or stars or happy faces. However, the school should not draw attention to the rocking motions to make the granddaughter feel uncomfortable. Just have them ask her if she needs to use the restroom just like they might ask the other kids. However when she does have a BM, she should really be praised!!!
With regards XXXXX XXXXX other children possibly making fun of your granddaughter, your granddaughter must be told that if she starts rocking, her classmates may notice and start asking her questions and might say unkind words. Therefore, she should use the restroom. However, many children even in elementary school refuse to use the restrooms at school. They like the comforts of the bathroom at home.
If your granddaughter has been checked out thoroughly by her pediatrician and there are no underlying medical conditions that are preventing her from having a BM, then I must tell you that the most probable reason why she is holding onto her BM is because it is more of a control issue. This is one of the last things that your granddaughter really has control over. and she does not want to give up that control.
Just do not make it a big deal. Ease up and never punish her about not having a BM. The more attention that is drawn to it, the worse it seems to get. What may happen is that she is trying to hold onto her BM as long as possible and then some may start to seep out staining her underwear, She will get really uncomfortable having stained underwear or much more uncomfortable with small pieces of BM that have slipped out. She may start having more BMs because she does not want to be embarrassed.
Some experts may tell you to add a small amount of mineral oil to juice to help the BM slide out because the longer she holds the BM, the harder it is to push it out and it becomes very uncomfortable and even a little scary.
Here are some books that might help:
It Hurts When I Poop: A Story for Children Who Are Scared to Use the Potty
By:Bennett and M.S. Weber
Where's The Poop?
By: Julie Markes
By: Taro Gomi
I Can't, I Won't, No Way!: A Book for Children Who Refuse to Poop
By: Tracey Vessillo
Other helpful hints are to include a fiber rich diet, keep up with the Miralax, and drink plenty of water.
Hope this was helpful. If you need further advice, please feel free to ask. Have an awesome rest of the week. Thank you for allowing me to assist you. :)