After much research, this is what we have finally decided upon due to many factors.

Playa Negra - 7 nights
Time to just enjoy some down time and beaches, private pool, Diria Natl Park, Ostional, etc. (chose this beach area based on cost, proximity to flying in to LIR, nearer to a hospital than southern Nicoya due to our daugher's medical concerns) We have 4 little kids....so we need some time to settle in and get in vacation mode!

Arenal/La Fortuna - 3 nights
We had so much trouble finding availability at rental homes and hotels that had accommodations large enough for the 6 of us. We happened upon what seems to be an AWESOME and UNIQUE experience....Leaves and Lizards has connections with a local family who is offering a brand new 4 BR home on a working dairy farm - 180 acres - kids can feed animals, milk cows, make cheese, prepare meals with the family, family speaks only Spanish (and we speak very very little!), etc. We really wanted 4 nights here but they didn't have the last night open - so we took what we could get! We are a homeschooling family and our kids love adventures and things like this - it just got us really excited! The house looks beautiful too!
While here, we also plan to to La Fortuna Falls, Proyecto Asis, Hanging Bridges, the free hot river near Tabacon, and Eco Centro Danaus. Not sure how we will fit it all in - but we will find a way!

Selva Verde - 1 night
We are still waiting to get confirmation on this. We had originally thought of doing a day trip here from Arenal if we were in Arenal longer. But, this way, we can pass through, spend a night to get a taste of the area for future trips and not worry about being so far from the airport on our last day. We ruled this out for our last three nights for a variety of reasons - but when we couldnt stay in Arenal for 4 nights, we thought we could at least get a little experience there and do maybe one guided hike - or possibly the wildlife boat ride (though I know volcanogirl said she didn't see any wildlife on the boat tour).
So, while here (if we can get it confirmed) we will likely do 1 guided hike and explore grounds on our own. Should we do hike at La Selva instead? Gotta do some more research here on tours good for little ones (maybe chocolate....maybe pineapple....???not sure). En route back down to Alajuela, we will likely stop at Toucan Rescue Ranch.

Alajuela / Vista del Valle area private home (3 nights) -
I know this isnt anyones favorite area....but for several reasons we made this our choice for the last 3 nights. It is nearby an orphanage in Atenas where we had visited and taken 3 large suitcases full of donations on a previous trip and would like to visit again. We can do Villa Blanca Cloud Forest area and Poas as day trips which we hadnt done before. And, the house is amazing and has private pool and hot tub for some relaxing prior to flying home....and it is only 30 minutes from the airport for our final morning!

So, we're pretty excited about it! Hoping to find a way to fit in all the activities we want to do! We will be there mid-late April! Not too long from now!

BTW - If Selva Verde can't accommodate us on that 1 night - any other suggestions for a cool stopover between Fortuna and Alajuela? (Villa Blanca requires us to take 2 rooms - so that doesn't work...).

Thanks for everyones help and feedback getting to this point!