This set of duplo would make a good starter set as it contains quite a few of the more fun items and fewer plain bricks. This set includes a couple of penguins, a polar bear and cub, a zookeeper, a fish and a tubular slide. The slide can be tricky for little ones to assemble, but once it's done they have great fun posting the penguins (and bricks) down and watching them shoot out of the other end.
As you would expect from duplo, it is really well made so it is very robust and there are no tiny parts to get lost. The only downside is that it is a little expensive, but what you get is chunky and well made and will still look as good as new when your children have outgrown it