Possible side effects to Splenda include:
bloating
abdominal pain
gas
nausea
diarrhea
headaches
migraines (severe headaches)
heart palpitations (fluttering, irregular heartbeats)
shortness of breath
depression or overwhelming anxiety
spaced-out or drugged sensation
joint pain
dizziness
I understand your interest in cutting down sugar intake, but I personally would not substitute an artificial product. My strong feeling is that children do best on a well-balanced diet of natural products (meaning no artificial or chemical additives or preservatives). Try shopping at Whole Foods or Trader Joe's if you have them in your area.
