California Holiday as family + maternity nanny - question...

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So my partner has been asked to speak at a conference in Arizona, aside from SXSW the past few years, this will be one of the largest crowds he has spoken to. He is on 3 months maternity leave but decided to do this talk since an old friend has asked him to.

I'm not a huge fan of traveling to America since I find it stressful, but I like Austin and California. We decided to take a family holiday to California for 1 week, my partner will fly to Arizona, then California to meet me, our baby and our maternity nanny who is flying from London with me.

We are planning on being on Venice Beach, however as much as I trust our nanny with the baby, I'm not sure how safe it is in general? I've never had problems but I'm nervous about taking my baby to Venice Beach, should it just be avoided?

Also, does America have Hipp Organic? We are currently supplementing since I have to regain my supply after being on antibiotics.

Could we get it from Whole Foods?

Because my partner is flying to Arizona and meeting me in California, would I need a letter from him to travel with our baby since he won't actually be with me? And will my baby need an ESTA? He is only going to be 2 months old.