Don't worry about it! Assume the best in them, especially if they have a well mannered kid. Walk up to them at the school event and just warmly introduce yourself and mention you sent the request and say you're still interested and realize they might have missed the message or been to busy to respond.

All is good! They are are probably very nice and just busy!

I would approach them at the event and introduce myself as "larlo's mom - you may remember me from the email I sent you about the playdate". and go from there.

I have had this experience esp with hispanic parents from our school

Assume positive intent. I would assume they saw it, wanted to reply, and then forgot. Approach them at the party and just ask again. Don't mention email.

You need to stop thinking of the world as revolving around you. One mom I know hasn't been responding to such emails as she has been in the hospital for the past month with a relative with critical, life threatening injuries. You never know what is going on in other people's lives and you should err on the side of thinking positive things about others.