Hemp heart alternatives
Walnuts are a source of ALA omega 3 fatty acid that is comparable to hemp seeds, however I find the taste a little too strong for smoothies and ice pops. You could try adding 1 tsp maple syrup for balance - although that would be pushing up the sugar. Almonds would make a fine mixture, but will not contribute ALA. Flaxseeds have more ALA than hemp, however I personally do not like the taste in smoothies. Your mileage may vary.
DHA oil products
I do not endorse any of the products cited here, I only mention them so you can do your own research. There are no affiliate links on the Vegan Family Kitchen web site but a quick search will help you find where to buy these.
Being in Canada, my preferred option is NutraVege. I use the 2X version (1000 mg per teaspoon) and only put 1 tsp in a batch of smoothies. It is also available on Amazon.com. Common choices in the USA are Dr. Fuhrman's (which is supposedly tasty) and about half a teaspoon would do. Barlean's Total Omega Vegan Swirl includes 130 mg DHA per tablespoon, plus lots of ALA and some EPA. Depending on how you like the taste, you may want to put a whole tablespoon in your smoothie batch. Finally, there is also Deva Nutrition Vegan Liquid DHA EPA, however I was not able to ascertain the concentration in long-chain omega-3s, so you can do the math and experiment.