Graham's playdate

Once a week I watch my friend Betsy's little girl Rebecca for a few hours in the morning. Graham loves when Rebecca comes over (I'm interpreting his emotions for him as talking isn't quite his thing yet). He watches her intently the entire time she is here, waiting to see what she will do next. I can't blame him since Rebecca is significantly more interesting then I am. She dances around, sings funny songs, and lays down next to him so they can share toys. It's pretty great. She can keep him occupied and happy for far longer then I have ever been able to. This week she ran circles around him until he was laughing hysterically and she was exhausted. Totally adorable. They also both share a love for Wonder Pets which is great because the opening song is so catchy.
Should I be concerned that Graham's head looks about twice as large as Rebecca's?