Is there a difference between "help them develop" and "help them to develop"?

Here are my two sentences.

1) Playing with children can help them develop their brains.

2) Playing with children can help them to develop their brains.

Also,

1) This medicine helps fetuses develop normal lung function.

2) This medicine helps fetuses to develop normal lung function.

Additionally,

Can I say, "Playing with children can help them develop their brain"?
rather than "...their brains"?

Thank you for your help!