With regards to MAGI, can states that want to have one eligibility level for children, ages 1-18, do so?

Yes. The new converted standards are based on the state's current income eligibility standards and their pre-2014 disregards. So if children in different age groups have different effective eligibility levels under a state's pre-2014 rules, the children will have different converted standards. For example, if a state has been covering children aged 1-5 to 133% FPL and children aged 6-18 to 100% FPL, the state's MAGI eligibility standard in 2014 may be 139% FPL for children aged 1-5 and 133% FPL for older children.