I'm suprised no one has yet suggested food allergies as the cause of the eczema on your 18 month old's body. Persistent cradle cap as a young baby and now this, plus eczema on face and in crooks of arms/legs just *throws up a big red flag* to me of food sensitivity or allergy. Probably one of the big 8: cow's milk, wheat, eggs, soy, peanuts, tree nuts, fish, shellfish.
I'd look at that list of 8 items and ask yourself "what on that list is my child absolutely wild about and wants to eat lots of every single day??" -- and then consider that item the most likely problem. Eliminate it completely from your toddler's diet for 4 weeks (including any hidden ingredients in any packaged foods, mixes, etc.), during which time all that eczema and cradle cap should clear up. If that happens but it's not convincing enough for you, you can always reintroduce the food after the 4 week elimination period and see how your child reacts, if the eczema returns, you'll know that's the food to avoid permanently. Reactions after reintroducing the food will usually show up within 72 hours.
If there's nothing on that list of the 8 most common allergens that your toddler loves, then consider if there's an item on that list that your toddler consistently *avoids* yet you still try to feed him - food allergic kids either tend to be avoidant of the food to which they're allergic OR they tend to be obsessed with it (almost addicted). Also consider your family history of food allergies... is there something that multiple people in your family are allergic to?
Anyway... eczema is not a normal thing that we or our kids just need to live with. It is an irritation that we see signs of externally (on skin) but that also exists internally, so it is really worth it to play detective, identify the cause, and remove it. Very often, eczema can be solved by identifying the allergen and removing it (whether it's a food allergy, environmental allergy like a pet, or a contact allergy like to a detergent or fabric softener). Creams and medications are just a temporary band-aid, but don't actually SOLVE the root cause of the eczema.
Best of luck in tracking down the skin issues - it might seem daunting at first to try and eliminate a particular food, but you can do it. You will quickly get good at reading labels. And google is a great source for finding that there are often easy substitutions (coconut milk instead of cow's milk, for example) --> it is all worth it, though, to track down the cause(s) of eczema and remove them from your toddler's system!