Well, when I was young I've played harmonica for two years, because I was late for signing up for flute that year. It went really good, but I didn't like that instrument, so I stopped with practice.
The next year there was an open space, so I played flute after that. I liked flute, and it was good for my voice.
After I finished flute, I went on solo singing, and I played piano, as a part of singing classes. Personally, piano is the most emotional, beautiful and sensitive instrument, I really love piano. I was also good at playing piano, but I didn't have one at home, so I didn't practice.
All of this being told, I want you to know that if you want those kids only to have fun you can choose whatever instrument you want. There is xylophone for example, it's easy to learn. But, if you want them to start learning and to keep playing for a longer time, there can't be only one instrument. If it works for the two of the children, for the two of them it won't.
Maybe you could get just a few different instruments and then see which fits to different child. Whatever you choose, good luck.