We have had a lot of strange things happen in every house we live in. In every house, it is young children, a little boy and a little girl. They dragged my mum out of bed one night. She is pretty convinced it is the spirits of two mcs she had years ago. They are never horrible, just playful. We are pretty sure they follow us from house to house and we don't mind. It sounds like your spirit could also be quite young.
In every house we have lived in we have had resident spirits aswell. In the house we are currently in, there is a tall man and he standing in the doorway between the living room and the hallway.
There is seriously no need to worry about it though, might just be a little kid who see's you with your little one and wants to play along, maybe someone who is missing their mother in spirit world. Also having a lo will spark up activity so thats probably why it has only just started happening. Also wait until your little one hits puberty, most spirits become active when there are teenagers.
Maybe try talking to the little one. If you want them gone, explain to them why and be sweet, like you were talking to a little boy or girl. If you don't mind them being there then tell them that, just tell them things like, not to bang when lo is asleep and stuff.
I might be prying and sorry if I am and this upsets you but have you had a mc or lost a child, if so it could be them coming back to look after their sister and trying to play along.
Just don't be scared, young spirits can be fun. They hide things just like a cheeky kid. They will never hurt you, they won't want to and they are not capable !