Hi Murgy, I too went to Beni for my 40th birthday and really enjoyed it.
Terra Mitica was only open on a weekend at that time but it's 4 years ago now.
I have been having a look on the website and it looks as though it opened on 24th March this year but closes Tuesdays and Wednesdays, I don't know if it will be the same next year.
This year we stayed in the Sol Pelicanos which seems to fit what you are looking for. It's a good hotel, very clean nice food e.t.c. very central near to bars restaurants and the beach and it also has a kiddies club but you would have to check to see if it ran at that time of the year.
I have been twice now in March and the weather has been quite good both times. mid 60's and generally sunny although chilly on an evening.
We went to the Benidorm Palace for my birthday which I would highly recommend, it was fabulous.
Hope you have a great time with whatever you decide.
Sue