LeapPad will be outgrown
A computer will not.
That is the bottom line.
I also suggest, whatever computer you get, get a MAC.
It is very user friendly, kids learn it easily, and it is intuitive.
Schools, use MAC computers for this reason.
And there are many good software, for kids per MACs.
My kids are 5 and 9.
Ever since they were Toddlers, they have just used what we have.
ie: Desk top computer (MAC), laptop (MAC), iPad, iTouch etc.
And they are very adept at it.
It also depends on your child's fine-motor skills and dexterity... per using a Laptop etc.
I would NOT buy a LeapPad.
I would buy, a MAC computer for them.
Now, as far as them BOTH using the computer at the SAME time.... no. I wouldn't even want to use the computer at the SAME time as my Husband. A child, will need to take turns. And there will be and may be, "fighting." That is just a given.
The 3 year old is also very developmentally and cognitively different, than a 5 year old.
One kid could be using the computer, while another kid is using an iPad.
My kids, from Toddlers, could use our computers. We taught them how.
And they use our iPad too.
In school, per my 5 year old son, they had even in Kindergarten, "computer class." With MAC computers. He is now in 1st grade, and the school has MAC computers and the iPads for the kids to learn on.
If you get a Laptop, I would get a mouse attachment. Then they don't have to use the finger pad... which is harder for some young kids.