Assalamu alaikum wa rahmatulahi wa barakatuh sister
This is an unfortunate situation where your ex husband had little to do with your children after you divorced, yet now you have remarried he is taking an interest in then by attempting to get them to go and live with his non-practicing parents.
The first thing I would suggest doing is getting some legal advice from someone who practices in shariah law to determine exactly what rights both you and he have regarding custody of your children as there are certain complexities to it that may or may not be compatible with what is normally practiced in your country. Allah has determined rules regarding custody that work in the children’s best interest so not is best to stick to these guidelines for the sake of everyone involved as well as being most pleasing to Allah. Knowing these rules alone will also make it easier to guide your choices about what to do next and if/how and when to tell your children. It will so give you legal support should any difficulties arise along the way.
Whatever the outcome is of the choice that is made, if you are able to talk to your ex husband under the correct circumstances, you could speak to him about making the process as easy as possible so as to make things more comfortable for the children without them having to be witness to argument’s or ill feelings.
Hopefully he will be agreeable to this, but if not, you can at least keep things comfortable on your side. You will be understandably stressed right now and it can be difficult to hide this from them, but managing your own levels of stress will help you to not let them feel it too. Children are very aware of these things and will often feel to blame for difficult times between parents, especially in the case of divorce, so it is important that amongst it all, whatever happens, that they don’t feel to blame for any of it.
Whatever the outcome, if it’s possible to arrange mutual times for contact, whether it be your going to visit them in the case that they must go and stay with their grandmother, or they visit your children if they are able to stay with you.
A way to make the process easier is to make it a gradual one. So, if it is that they must go, perhaps you could begin by making visits to the grandmother for a few hours then the day, then overnight. This will also make it easier to tell them that they are going to live there as they will have gotten used to her and the house over time. Beyond this, in sha Allah you can arrange regular visits to them also. Or, if it is you that will maintain custody, you could still allow them to stay with their grandmother also and this way perhaps they will see more of their father.
If it is ruled that they must go then you need to consider if or how to tell then. As mentioned above, a gradual introduction will make this easier, but consider the emotions that they will face going to live away from home after being with you the entire time to this point. Of course, you don’t have to tell them, but this could come at a cost in how they feel in that you kept it from them, but at the same time you may feel this is the best way to protect them and make the process quicker and easier. Weigh up your options and the consequences and ask Allah to guide you through istikhara. At least, once you have the shariah guidance on the matter it will be a lot easier to answer their questions as your responses will come straight from understanding Allah’s way.
May Allah guide you to do what is right and what is most pleasing to Him. May He make the whole process smooth and easy for all whatever the outcome is.