This is a two part question.
- Can summons benefit from magical healing?
- Can Eidolons benefit from magical healing? This question treats normal and unchained summoners as equal unless there is a specific difference where it concerns healing.
I have looked at the summon monster spell and it does not say that the monster summoned actually exists (for instance the eidolon specifically states that its an aspect of the creature and not the creature itself) or if its an aspect of a greater creature.
Reading cure light wounds it talks about what happens with living and undead, but nothing else is mentioned.
Summons may be healed using normal curative spells (unless you are using some weird summoning variant that disallows that, but Summon Monster/SNA/etc. are all healable) assuming their type is usually subject to magical healing. (Summoned undead, constructs, etc. all heal or not as usual.)
Eidolons don't heal naturally but can be cured with normal healing spells. Summoners don't get those spells (because then they'd be healbots in addition to ruling in all other ways) but they do get Rejuvenate Eidolon - not because normal healing doesn't work on eidolons but because you don't want to give summoners general purpose healing.
Unless you find a rule specifically saying something is an exception, then they always fall under general rules. Everything is healable via cure spells unless they have a rule about themselves or their type or class or whatever that says explicitly that they are not.