You say family, children, young, teens?? It would be a bearing on what I would suggest. Also budget for the likes of hotels etc.
What about a 2 centre (dont know how long your wanting to be away) Mainland Malaysia then say Malaysian Borneo. Or maybe Singapore then Malaysia which has some lovely small islands to stay on the East coast and April is a good time, but will be hot and humid.
Thailand has lots to offer but in April it will be very hot in many places.
Sri Lanka, that time is between the monsoon season but crowds will be starting to get thinner, but the weather would be fine, but with rains starting. Prices are rising for accommodation (and vat has just gone up), however if you do decide to investigate further, do not use Sri Lankan Airlines for flights, lots of cancellations and delays last minute. But you do have everything from beaches to hill stations to wild elephants.
Maldives, weather should be ok its just outside of the monsoon season, but hot and the prices are even higher due to Easter, choose carefully as it can get stupidly expensive (and the add ons of boats/flights) you can get your eyes taken out for some places.
Not a fan at all of Vietnam.
Out of them all I would choose Malaysia.