You’re not ‘about to have a heart attack’.
You’ve had an ecg and if it had shown anything concerning your doctor would have been in touch urgently.
Regarding what happens to your child if you’re taken by ambulance and you have no childcare:
They’ll suggest neighbours first if no family or friends available.
Ambulance will call social services. Social services will either arrive at location and take your child into temporary foster care,
or if there’s no time, ambulance staff will take child with you in ambulance and liase with social service at hospital (there will be a dedicated staff member who will take care of child until social services arrive).
Or ambulance staff will call police (probably policewoman if that’s any reassurance) who will arrive with urgency to take supervision of your child until social services liase with them.
I had gastroenteritis as a lone parent with a baby and toddler and ambulance was called. At no point would they leave a child unsupervised at home whilst they take you in by ambulance. The police and social services have staff on emergency call-outs for exactly this sort of thing, don’t worry 
Calm yourself with gentle breathing exercises tonight, or call 111 for reassurance, and contact your doctor first thing in the morning.
Take care, I have been there, and I’m sure others on this post have too. Everything will be alright. Ring 111 just hearing a warm, professional and knowledgeable human will help hugely with any anxiety.