I'd say that you were at fault. Big time I'm afraid, but lets just be glad that you and your child were not injured or killed.
The lorry could have crushed you both. They need a lot of space to make turns safely, especially if it is an articulated truck with a long trailer. They have enormous blind spots and cannot look into all of them fully when out manoeuvring on the road, especially up the inside on the left. That is why many do now have notices printed on the back of them to remind other drivers and cyclists.
As for whether it was indicating or not, maybe it was or maybe it wasn't. Lorries' indicators can often be in the very top and/or bottom corners of the rear of the trailer, so not always the easiest to spot. You really need to have your wits about you.
The lesson to take from this is to give them space, and NEVER go up the inside of them. If it turns left then the trailer can clobber you and crush you. If it turns right then the trailer may still initially swing in towards you in a way you might not be anticipating. All because of the amount of room they need to make the turn.
You've learned a harsh and expensive lesson this morning and I am very glad everyone is OK. You will learn from it and you won't make that mistake again.
There are times in everyone's driving career when we do something daft and realise how lucky we have been and how a guardian angel must have been watching over us that day. Luck and guardian angel were with you today so be glad of that.