"Every child has an unlimited potential", said Dr Shinichi Suzuki.
So, how do we motivate children to do their work - practice their instrument, do their homework, help in the house etc.
Do we need to motivate them?
Anything we learn needs encouragement and acknowledgement, in order to make us want to repeat it. 
When we want our children to have the desire to do learn, play and practice, we have to reflect on  our role in it. 
Love and desire are necessary  to acquire ability. 
The thirst to learn is innate in every child. We don't need to create it, but we need to make sure we don't harm it. 
How can we allow it to grow and continue to develop?
Self motivation might be the key here. Self motivation comes with self discipline. 
When children become self motivated it transfer to other areas in their life. It becomes a part of their character.
" Man is a son of his environment", Shinichi Suzuki.
If art and  music are in the environment , children will naturally  absorb the love for it. 
When a parent asks, "how can I help my child love doing this", the answer will be, " love it yourself".  
The way you approach the practice will be the way your child approaches the practice. 
Like what you want your child to like. Invest the love, and trust that it will be in the heart of your child. 

One of my favourite quotes:
“A seed is planted in the earth. We don’t see when the germination begins.That is the doing of Mother Nature. We have to wait patiently.We cannot dig up the seed to see wheatear it is really growing; to do so would be to destroy everything. Suddenly a bud appears. What a joy and pleasure to watch it grow! At the same time the root, unseen in the ground, is getting stronger and has the power to produce a big, sturdy tree”. (Shinichi Suzuki, Nurtured by Love)

Love and nurture the seed with curiosity and wait to marvel in the flower or tree. 
Don't judge day by day. keep the big picture in your heart. Believe in your child. 

You could listen to this topic on The Souloist Podcast episode  #51
​