"My son and I are at the park."
Translation:Mi hijo y yo estamos en el parque.
You use somos for characteristics or identity, and estar for states of being or for location. Don’t worry too much about whether a thing is temporary or permanent. For instance, you always use estar for location, even when talking about permanent locations. (Dónde está Canadá?)
How you feel or where you are, don’t forget to use estar!
My son and I = We = Somos/Estamos
Estamos is used in temporary states such as emotion or location. Somos is used when a state is more permanent in nature.
In this case "Estamos" should be used because they are stating their location.
Example using both at the same time: Esp:"Mi hijo y yo somos rubios así que estamos en el parque" Eng:"My son and I are blond so we are in the park".
Being blond-haired is considered a permanent state, so they would use "somos" for that, but their location isn't permanent so "estamos" was used.
Hopefully that helps. :)
A modification: Please ignore the temporary/permanent thing. It can lead you astray.
Ser is used for characteristics and occupations.
Estar is for condition or location --- including locations that are (probably) permanent. Remember the sentence in one of the early lessons? Dónde está el banco? Of course there's more about ser amd estar in whatever resource you use.
Ser conjugations are used for characteristics and occupations. Estar conjugations are for condition or location. You'd use nosotros somos for sentences like "We are both tall," or "We are teachers."
Here's the complete list of uses of the two forms of to be.
SER ("DOCTOR") Date Occupation Characteristics Time Origin Relation
ESTAR ("PLACE") Position Location Action Condition Emotion