The same?

[fluffyhamster]

Can these two sentences absolutely be taught together, or would it be better if they received somewhat differing treatments?

Here's a picture of somebody playing tennis.

Who's the/that boy sitting under the tree?

[metal56]

Can these two sentences absolutely be taught together, or would it be better if they received somewhat differing treatments?

Here's a picture of somebody playing tennis.

Who's the/that boy sitting under the tree?

Not better, not worse, IMO. It depends on what you want the student to focus on.

EG

Sentence heads.

Here's a + noun

Here's a man who knows what he's talking about.
Here's a penny for your thoughts.
Here's another question from Fluff.

No reason to teach the other sentence in that context.

[Anuradha Chepur]

Here's a picture of somebody (who is) playing tennis.

Who's the/that boy (who is) sitting under the tree?

There is an omitted subject (relative pronoun) plus auxiliary in both.
Probably you can teach them together.

[fluffyhamster]

Yes, "omitted" RPs/"reduced" relative clauses. But how about when a proposition/info that e.g. "Bob (is/=) playing tennis" is expressed thus: 'I saw Bob playing tennis'? Or 'I saw a man walk(ing) his dog (all day)' etc.

[lolwhites]

I saw Bob playing tennis sounds ambiguous to me. Probably Bob was playing tennis and not the speaker, but it reminds me of when my schoold headmaster angrily announced "I saw a sandwich walking down the corridor!"