Vikingstaff
Moderator
Congratulations on the new boat order. With it being a done deal, just reveal in what you spec'd up! 
With your 20 mph limit on the lake, I'd think you would be fine at 70, or even 90hp - particularly if the grandkids are younger.
If you thought you might tow it to other lakes, or wanted to future proof it for tubing as the grandkids get older, I'd then say move up on engine hp. That said, I am about to be a new pontoon owner myself AND I am HP biased in general...cars, boats, running, walking, etc... Hence, its probably better not to listen to me.
With your 20 mph limit on the lake, I'd think you would be fine at 70, or even 90hp - particularly if the grandkids are younger. If you thought you might tow it to other lakes, or wanted to future proof it for tubing as the grandkids get older, I'd then say move up on engine hp. That said, I am about to be a new pontoon owner myself AND I am HP biased in general...cars, boats, running, walking, etc... Hence, its probably better not to listen to me.