We have a family of 5, DS-9, DD-5, DS-2, and I now bring our 2-ways on any substantial family outing.....definitely to WDW. Due to the differences in ages, we do split up occasionally, and the radios are really needed. Before we had the radios, we had an incident where my husband and I split up with the different kids and agreed to meet by the Tomorrowland Transit Authority (TTA) when my husband and 2 older kids were done doing the Haunted Mansion. He got to the Haunted Mansion and found it broken, so he headed over to TTA. I was hanging out in that area as well after doing a diaper change. After an hour and half, we finally found each other.....he had been just across the 'traffic pattern' from me (i.e. about 40 feet away) the entire time and we never saw each other!
I would think even with your kids being 2-3 years apart you'll want to split up sometimes. If you both have cell phones that might be fine (although I have heard that some cell phones don't work well in Disney.)
If you do buy radios, be sure to buy some that have the sub-channels; otherwise, my understanding is there's just too much traffic to use them effectively in a busy place. Have a great trip!