Assuming tank is 39 x 28 x 32 high, that’s limited swimming space so be thoughtful in your fish choice, eg, most tetras would prefer a 60cm swimming length - look for less active fish that remain quite small
Guppy males (look for strains that are smaller) BUT guppies often have quite short lifespans (so be prepared for this)
Guppy x Endler would be my preference - these are still very colourful but remain petite (usually less full finnage than pure linebred guppy strains)
Endler - while some of these can be very colourful, especially Endler x Guppy (they have fewer guppy genes than the above crosss), your kiddo may be less impressed
Given the tank size, I’d definitely avoid adding any females unless you’ve another tank ready for the multiples
Shrimp are definitely a lot of fun and much more visible if no fish are in the tank
If you add some guppy (or endler) they will likely take some of the shrimp babies, as long as they pay little attention to the older shrimp, their addition won’t impact shrimp visibility too much
(Guppy/Endler females will definitely show shrimp more attention than they’d prefer)