How do you strengthen their core muscles? My LO hates tummy time and sitting up and mostly wants to be held in a standing position and bouncing himself. He will lay down to sleep and on the play mat and sit in the swing but HATES tummy time no matter what I do. I'm worried he's not going to meet his milestones. Every time I put him on his tummy he rolls out immediately! He also hates if I put him in a sitting up position so I'm worried he won't be able to sit up on his own. He's 14wk, any suggestions would be sooo much appreciated!