I think that in the sentence that brought up Siraisi Tomio words "could"
and "can" do not really matter. What really matters, in my opinion, is word
"would."
If we will rephrase this sentence and say: The little child is lonely; if he
had someone that he can play with, he would be happier. To me, this is the
"normal" subjunctive form of  sentence. Whether child "could" or "can"
doesn't matter at all, but what really matters is that child "would be
happier" if...  Especially, "could" or "can" doesn't matter because it
follows the word "that" which creates in this case a relative clause. But I
would've replaced word "that" with word "whom" in order to make relative
clause more clear. I'll be happy to get any comments about my opinion
regarding subjunctive mood. Thank you very much.
Respectfully, Aram.