With infants, make your presence known by narrating each activity that you do together. Say "I am bouncing you on my knees" or "you are holding your bear" or "we are reading a book". If your baby is uninterested in an activity, talk them through moving to the next activity, "we are finished reading a book, let's talk a walk outside!".
With Toddlers, talk and model how to play with one another: "We are playing with these duplos together", "Can I have the green car when you are done?". Encourage your toddler when they exhibit positive behavior "thank you for giving me a turn with the car. It makes me happy when you share."