Hi Mommie K.!
It use to work when my kids were that age.
I was a working mom, the sitter fed them (heavy)meals, early in the evening 5PM -6Pm. Let them play, shower and then get ready for bed by 8PM. Believe it or not: the kids then are grown now, they still stick to that with their own, my younger one lives off-island she's in bed by 10:00 PM.
I believe it's a routine that mom(s) have to stick with early on, for when the kids are little, it is easier to mold,teach or practice.
One more thing; No sweets after dinner, if you must then givein to fresh fruits. If/when they are already in bed and can't sleep, warm up milk, give a small portion, turn off the lights leave the b/room door slightly open(assure quietness). Do not have the T.V. 'on' in their room or close by so they can hear someone is still up.
Did I mention, if baby sleeps w/other sibblings -be a copy-cat, but if he sleeps with mom-mom would have to go nite-nite too.
Oh I almost forgot no sweet snacks