I will assume you're fortunate enough to have a more healthy family system and not a dysfunctional one as OP describes.
OP, look up the narcissistic family cult. See if any of it resonates. See if granny dearest might fit the role of the narcissistic matriarch. I have a feeling she will.
When those seeking a healthier dynamic go no contact, it's for one reason; protection. To protect from abuse, manipulation and harm usually after a lifetime.
If the unhinged ones do it, it's a game, a punishment or you've been entirely discarded because as an object to them, they feel they can't manipulate you any longer.
I'd say this is a game. I'd guess they'll be back and you're at risk of being sucked in with false love bombing.
This is a favour to you. The time away from them will give you chance to really assess the relationship. I'd read up on everything I've suggested and start lifting the veil.
Your 3 year old is at risk of significant toxicity and manipulation so the more detachment the better. It's just very tough.
At 3 they're going to be diverted away from talk of granny dear quite quickly. Very basic excuse will suffice for now and just repeat the same line if they ask.
It's harder for you all this. The silent treatment right now is to punish you, to control you, to set off insecurity and anxiety ( which is working) and then when they re contact you're almost grateful. Because you're still bonded to them. They give you enough niceness to keep a part of you attached.
Healing comes through no contact and sticking to that. It's incredibly difficult and I don't underestimate how hard it is to accept what I've said here.