Between these dogs… wich one is better?

None of those dogs are friendly towards kids.
Little dogs normally lock onto one person and won't like anyone else.

All of those dogs require extreme grooming.
The yorkie doesn't shed.
References :

Out of those dogs I guess I would say West Highland White terrier. The ones I know love everyone they meet and are very smart. You can trim their hair down shorter too. It might be a challenge to keep their hair white though.
References :

I think you mean west highland white terrier or westie for short

None of those breeds are recommended for kids, they all require extensive grooming and equal care.
References :

Toy breed and small dogs are not typically 'easy to train'. They essentially do everything for them, not for you.

"Friendly towards kids & elders" is an aspect of proper breeding, the individual dog, proper socialization, proper training of the dog, and proper training of the children. No one breed is better than another, but of those, the Bichon is probably the most tolerant of kids & elders.

All of those breeds have extensive grooming requirements. Daily to weekly brushing on top of visits to the groomers every 4-6 weeks. The Pom will have coat blows twice a year and both the Pom and Westie will drop hair year round.
References :

Terriers tend to be harder to train than some dogs (this is of course a generalization, all dogs are different, but this tends to be common). If you want a dog that is friendly towards kids and elders, I would either suggest getting an adult dog from a reputable breeder (http://www.jlhweb.net/Boxermap/reputablebreeder.html/), or get an adult dog from a shelter or rescue group that does temperament testing (www.petfinder.com), a puppy may be a 'surprise package', even from a reputable breeder, and needs a LOT of care, training, and socialization, more socialization than and time usually than an adult dog.
References :

Bichons need very specific grooming needs and are very expensive to groom and it just makes a mess. Yorkies need trims, too, monthly. Poms and Westies need very good groomers, they are constant shedders. So Yorkie probably needs the less work grooming-wise

The easiest dog to train: Probably the Pomeranian, ONLY because they rank higher (23) most intelligent dogs list, however intelligent doesn't always mean easy to train

Dog must be friendly towards kids and elders: Depends COMPLETELY on how it's raised. If it is raised badly, it will act badly towards everyone. If it is raised well, it will love people.

My suggestions for grooming:
Bichon- Bichon Cut
Pomeranian- Teddy Bear cut
Westie and Yorkie- Puppy Cut

Your groomer should know what these are and ask some basic questions about how you would like them to be groomed if you ask for these.

I hope you choose adoption
References :

I would pick the "westie". Socialization would solve social issues. I think that they might need less grooming. Remember:small dogs and small kids don't make a good mix.
References :

scroll down on these sites and look at the star graph:

Bichon Frise:
http://www.breederretriever.com/dog-breeds/106/bichon-frise.php

West terrier:
http://www.breederretriever.com/dog-breeds/229/west-highland-white-terrier.php

Yorkshire Terrier:
http://www.breederretriever.com/dog-breeds/233/yorkshire-terrier.php

Pomeranian:
http://www.breederretriever.com/dog-breeds/194/pomeranian.php

Also have you ever checked out the Papillon there known for being easily trained and affectionate! they also only need mild grooming and little exercise.

Papillon:
http://www.breederretriever.com/dog-breeds/185/papillon.php
References :