I have no recommendations on actual hair products...that will depend on your childs hair type, your budget and time contraints for the routine.
My daughter has always had long hair. Thick and heavy...curls as a toddler and small child, that became waves and then mostly straight as the weight grew into it.
The brushing routine was awful for us! But she did not want to cut it....she was very singleminded as a 2 year old (and still is as a teenager!)! So I wanted to do what I could to maintain it. A friend recommended a BOARS HAIR bristle brush for her hair when it was dry. This was the BEST thing on the planet for us! We used a spiny brush or a wide toothed comb for the wet hair, plus a spray leave in conditioner (usually not a child specific product and generally not a super high end product, but something like a Fantastic Sams sort of product worked well for us.. a few squirts was plenty).....her hair was shampooed every other day in general...of course depending on activity. She usually bathed at night and went to bed with damp hair, but we did what we called a "loose" braid for bedtime..a very loose braid that just kept the hair together so she didnt get that awful rats nest from sleeping which would make for a much more difficult and painful for her grooming session in the morning.
But the BOARS HAIR bristle brush for dry hair was our best #1 product for maintaining the long long hair!
Good Luck~