Baby Bicycle Helmet Bicycle Safety?

Bicycle safety? - baby bicycle helmet

I recently bought a bicycle with a rear mounted child seat. I used bought in a secondhand shop, so they have no security for the seat.

What is the time that the child before he could get? My 7 months, is on his own and is very strong. But I can not find a helmet for a child under 3 years. I tried Walmart and Toys'R'us.

Can I take my daughter to the seat? Do you have helmets for babies? If so, where can I a find?

Thank you for any help!